Вызвано: java.lang.NullPointerException: невозможно передать значение null для аргумента pathString в child ()

Я пытаюсь получить данные на RecyclerView из Firebase. Но я испытываю NullPointerException. Я ввел детей, необходимых для получения данных, но не могу. Что могло быть не так? Ниже мой код ...

ChatActivity

package com.dreamlazerstudios.gtuconline;

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.text.TextUtils;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.TextView;

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ServerValue;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class ChatActivity extends AppCompatActivity {

private DatabaseReference rootRef, messagesRef;
private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;
private String userID;
private String mChatUser;

private ImageButton chat_add_btn, chat_send_btn;
private EditText enter_message;

List<DataSnapshot> listData;
RecyclerView recyclerView;
ChatActivity.MyAdapter adapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_chat);
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    recyclerView = findViewById(R.id.messages_list);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    recyclerView.setHasFixedSize(true);

    messagesRef = FirebaseDatabase.getInstance().getReference();

    listData = new ArrayList<>();
    adapter = new ChatActivity.MyAdapter(listData);
    adapter.setHasStableIds(true);

    chat_add_btn = findViewById(R.id.chat_add_btn);
    chat_send_btn = findViewById(R.id.chat_send_btn);
    enter_message = findViewById(R.id.chat_message_view);

    loadMessages();

    rootRef = FirebaseDatabase.getInstance().getReference();
    mAuth = FirebaseAuth.getInstance();
    final FirebaseUser user = mAuth.getCurrentUser();
    userID = user.getUid();

    mChatUser = getIntent().getStringExtra("user_id");
    String username = getIntent().getStringExtra("user_name");
    setTitle(username);

    rootRef.child("Chat").child(userID).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            if (!dataSnapshot.hasChild(mChatUser)) {
                Map chatAddMap = new HashMap();
                chatAddMap.put("seen", false);
                chatAddMap.put("timestamp", ServerValue.TIMESTAMP);

                Map chatUserMap = new HashMap();
                chatUserMap.put("Chat/" + userID + "/" + mChatUser, chatAddMap);
                chatUserMap.put("Chat/" + mChatUser + "/" + userID, chatAddMap);

                rootRef.updateChildren(chatUserMap, new DatabaseReference.CompletionListener() {
                    @Override
                    public void onComplete(DatabaseError databaseError, DatabaseReference databaseReference) {
                        if (databaseError != null) {

                            Log.d("CHAT_LOG", databaseError.getMessage().toString());

                        }
                    }
                });
            }

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

    chat_send_btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            sendMessage();
        }
    });

}

private void loadMessages() {
    messagesRef = FirebaseDatabase.getInstance().getReference().child("gtuconline").child("messages").child(userID).child(mChatUser);

    messagesRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            Messages messages = dataSnapshot.getValue(Messages.class);

            listData.add(dataSnapshot);
            recyclerView.setAdapter(adapter);
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}

private void sendMessage() {
    String message = enter_message.getText().toString();
    if (!TextUtils.isEmpty(message)) {

        String current_user_ref = "messages/" + userID + "/" + mChatUser;
        String chat_user_ref = "messages/" + mChatUser + "/" + userID;

        DatabaseReference user_message_push = rootRef.child("messages").child(userID).child(mChatUser).push();

        String push_id = user_message_push.getKey();

        Map messageMap = new HashMap();
        messageMap.put("message", message);
        messageMap.put("seen", false);
        messageMap.put("type", "text");
        messageMap.put("time", ServerValue.TIMESTAMP);

        Map messageUserMap = new HashMap();
        messageUserMap.put(current_user_ref + "/" + push_id, messageMap);
        messageUserMap.put(chat_user_ref + "/" + push_id, messageMap);

        enter_message.setText("");

        rootRef.updateChildren(messageUserMap, new DatabaseReference.CompletionListener() {
            @Override
            public void onComplete(DatabaseError databaseError, DatabaseReference databaseReference) {
                if (databaseError != null) {
                    Log.d("CHAT_LOG", databaseError.getMessage());
                }

            }
        });
    }
}

public class MyAdapter extends RecyclerView.Adapter<ChatActivity.MyAdapter.ViewHolder> {

    List<DataSnapshot> list;

    public MyAdapter(List<DataSnapshot> List) {
        this.list = List;
    }

    @Override
    public void onBindViewHolder(@NonNull final ChatActivity.MyAdapter.ViewHolder holder, final int position) {

        final DataSnapshot studentSnapshot = list.get(position);

        final Messages students = studentSnapshot.getValue(Messages.class);

        assert students != null;
        holder.messageText.setText(students.getMessage());


    }

    @NonNull
    @Override
    public ChatActivity.MyAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.message_single_layout, parent, false);

        return new ChatActivity.MyAdapter.ViewHolder(view);
    }

    public class ViewHolder extends RecyclerView.ViewHolder {

        TextView messageText, timeText, userName;

        public ViewHolder(View itemView) {
            super(itemView);

            messageText = itemView.findViewById(R.id.user_single_message);
            timeText = itemView.findViewById(R.id.user_single_time);
            userName = itemView.findViewById(R.id.user_single_name);

        }

    }

    @Override
    public int getItemCount() {
        return list.size();
    }
}


}

Мой модельный класс

package com.dreamlazerstudios.gtuconline;

/**
* Created by Gabriel Hagan on 23/05/2018 at 17:18.
*/
public class Messages {

private String message, type;
private long time;
private boolean seen;

public Messages(String message, String type, long time, boolean seen) {
    this.message = message;
    this.type = type;
    this.time = time;
    this.seen = seen;
}

public Messages() {
}

public String getMessage() {
    return message;
}

public void setMessage(String message) {
    this.message = message;
}

public String getType() {
    return type;
}

public void setType(String type) {
    this.type = type;
}

public long getTime() {
    return time;
}

public void setTime(long time) {
    this.time = time;
}

public boolean isSeen() {
    return seen;
}

public void setSeen(boolean seen) {
    this.seen = seen;
}
}

Мой макет xml

<?xml version = "1.0" encoding = "utf-8"?>
<RelativeLayout xmlns:android = "http://schemas.android.com/apk/res/android"
xmlns:app = "http://schemas.android.com/apk/res-auto"
xmlns:tools = "http://schemas.android.com/tools"
android:layout_width = "match_parent"
android:layout_height = "match_parent"
tools:context = ".ChatActivity">

<android.support.v7.widget.RecyclerView
    android:id = "@+id/messages_list"
    android:layout_width = "match_parent"
    android:layout_height = "match_parent"
    android:layout_above = "@+id/text_layout">

</android.support.v7.widget.RecyclerView>

<LinearLayout
    android:id = "@+id/text_layout"
    android:layout_width = "match_parent"
    android:layout_height = "50dp"
    android:orientation = "horizontal"
    android:layout_alignParentBottom = "true"
    android:weightSum = "100">


    <ImageButton
        android:id = "@+id/chat_add_btn"
        android:layout_width = "0dp"
        android:layout_height = "match_parent"
        android:layout_weight = "20"
        android:background = "@color/white"
        android:contentDescription = "@string/image"
        android:src = "@drawable/ic_add_black_24dp" />

    <EditText
        android:id = "@+id/chat_message_view"
        android:layout_width = "0dp"
        android:layout_height = "match_parent"
        android:layout_gravity = "center"
        android:layout_weight = "60"
        android:background = "@color/white"
        android:ems = "10"
        android:hint = "Send a message"
        android:inputType = "none"
        android:paddingEnd = "3dp"
        android:paddingStart = "3dp"
        android:paddingTop = "5dp"
        android:textSize = "17sp" />

    <ImageButton
        android:id = "@+id/chat_send_btn"
        android:layout_width = "0dp"
        android:layout_height = "match_parent"
        android:layout_weight = "20"
        android:background = "@color/white"
        android:contentDescription = "@string/send"
        android:src = "@drawable/ic_send_black_24dp" />


</LinearLayout>

</RelativeLayout>

Макет для recyclerview

<?xml version = "1.0" encoding = "utf-8"?>
<LinearLayout
xmlns:android = "http://schemas.android.com/apk/res/android"
android:layout_width = "match_parent"
android:weightSum = "1"
android:layout_height = "wrap_content">

<LinearLayout
    android:layout_width = "match_parent"
    android:layout_height = "wrap_content"
    android:orientation = "vertical"
    android:layout_marginTop = "5dp"
    android:padding = "2dp">

    <LinearLayout
        android:layout_width = "wrap_content"
        android:layout_height = "wrap_content"
        android:orientation = "horizontal">

        <TextView
            android:textStyle = "bold"
            android:id = "@+id/user_single_name"
            android:layout_width = "wrap_content"
            android:layout_height = "30dp"
            android:layout_marginStart = "10dp"
            android:layout_marginTop = "10dp"
            android:text = "@string/full_name"
            android:textColor = "@android:color/background_dark"
            android:textSize = "16sp"/>

        <TextView
            android:id = "@+id/user_single_time"
            android:layout_width = "60dp"
            android:layout_height = "30dp"
            android:layout_marginTop = "10dp"
            android:text = "00:00"
            android:textColor = "@android:color/background_dark"
            android:layout_marginStart = "10dp"/>

    </LinearLayout>

    <TextView
        android:id = "@+id/user_single_message"
        android:layout_width = "wrap_content"
        android:layout_height = "wrap_content"
        android:layout_marginBottom = "5dp"
        android:layout_marginStart = "10dp"
        android:text = "Message Text"
        android:textSize = "12sp" />

    <View
        android:id = "@+id/first_view"
        android:layout_width = "match_parent"
        android:layout_height = "1dp"
        android:layout_marginTop = "5dp"
        android:layout_marginStart = "2dp"
        android:layout_marginEnd = "2dp"
        android:background = "@color/lightgrey" />

</LinearLayout>



</LinearLayout>

Честно говоря, не знаю, в чем может быть проблема. Может быть, у кого-то есть орлиные глаза, чтобы определить ошибку и помочь мне ее решить.

Стек трассировки проблемы выглядит следующим образом.

05-24 07:51:43.697 19173-19173/com.dreamlazerstudios.gtuconline E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.dreamlazerstudios.gtuconline, PID: 19173
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dreamlazerstudios.gtuconline/com.dreamlazerstudios.gtuconline.ChatActivity}: java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2984)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
        at android.app.ActivityThread.-wrap14(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6776)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
     Caused by: java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
        at com.google.firebase.database.DatabaseReference.child(Unknown Source)
        at com.dreamlazerstudios.gtuconline.ChatActivity.loadMessages(ChatActivity.java:121)
        at com.dreamlazerstudios.gtuconline.ChatActivity.onCreate(ChatActivity.java:67)
        at android.app.Activity.performCreate(Activity.java:6956)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1126)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2927)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045) 
        at android.app.ActivityThread.-wrap14(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6776) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386) 

Это предыдущее действие, которое я получаю от

package com.dreamlazerstudios.gtuconline;

import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.squareup.picasso.Picasso;

import java.util.ArrayList;
import java.util.List;

import de.hdodenhof.circleimageview.CircleImageView;

import static android.content.ContentValues.TAG;

public class StudentsList extends AppCompatActivity {

DatabaseReference databaseReference;
ProgressDialog progressDialog;
List<DataSnapshot> listData;
RecyclerView recyclerView;
StudentsList.MyAdapter adapter;

private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;
private DatabaseReference myRef;
private FirebaseDatabase mFirebaseDatabase;
private String userID;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_students_list);
    setTitle("List of Students");

    recyclerView = findViewById(R.id.students_list);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    recyclerView.setHasFixedSize(true);

    listData = new ArrayList<>();
    adapter = new StudentsList.MyAdapter(listData);
    adapter.setHasStableIds(true);

    GetDataFirebase();

    progressDialog = new ProgressDialog(this);
    progressDialog.setMessage("Loading Data...");
    progressDialog.show();

    mAuth = FirebaseAuth.getInstance();
    mFirebaseDatabase = FirebaseDatabase.getInstance();
    myRef = mFirebaseDatabase.getReference();
    final FirebaseUser user = mAuth.getCurrentUser();
    userID = user.getUid();

    mAuthListener = new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            FirebaseUser user = firebaseAuth.getCurrentUser();
            if (user != null) {
                // User is signed in

                Log.d(TAG, "onAuthStateChanged:signed_in:" + user.getUid());
            } else {
                // User is signed out
                Log.d(TAG, "onAuthStateChanged:signed_out");
            }
            // ...
        }
    };

}

void GetDataFirebase() {

    databaseReference = FirebaseDatabase.getInstance().getReference().child("Users").child("Students");

    databaseReference.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {

            Students students = dataSnapshot.getValue(Students.class);

            listData.add(dataSnapshot);

            recyclerView.setAdapter(adapter);

            progressDialog.dismiss();
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });


}

@Override
public void onStart() {
    super.onStart();

    mAuth.addAuthStateListener(mAuthListener);

}

public class MyAdapter extends RecyclerView.Adapter<StudentsList.MyAdapter.ViewHolder> {

    List<DataSnapshot> list;

    public MyAdapter(List<DataSnapshot> List) {
        this.list = List;
    }

    @Override
    public void onBindViewHolder(final StudentsList.MyAdapter.ViewHolder holder, final int position) {

        final DataSnapshot studentSnapshot = list.get(position);

        final Students students = studentSnapshot.getValue(Students.class);

        final String list_user_id = studentSnapshot.getKey();

        holder.news_topic.setText(students.getName());
        holder.news_body.setText(students.getProgramme());

        if (students.getOnline() == true) {
            holder.online.setVisibility(View.VISIBLE);
        } else {
            holder.online.setVisibility(View.INVISIBLE);
        }

        Picasso.with(holder.news_image.getContext()).load(students.getThumb_image()).placeholder(R.drawable.student_icon_17870).into(holder.news_image);

        holder.itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent chat_intent = new Intent(StudentsList.this, ChatActivity.class);
                chat_intent.putExtra("user_id", list_user_id);
                chat_intent.putExtra("user_name", students.getName());
                startActivity(chat_intent);
            }
        });

    }

    @NonNull
    @Override
    public StudentsList.MyAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.users_list_layout, parent, false);

        return new StudentsList.MyAdapter.ViewHolder(view);
    }

    public class ViewHolder extends RecyclerView.ViewHolder {

        TextView news_topic, news_body;
        CircleImageView news_image;
        ImageView online;

        public ViewHolder(View itemView) {
            super(itemView);

            news_topic = itemView.findViewById(R.id.user_single_name);
            news_body = itemView.findViewById(R.id.user_single_status);
            news_image = itemView.findViewById(R.id.user_single_image);
            online = itemView.findViewById(R.id.online_status_icon);

        }

    }

    @Override
    public int getItemCount() {
        return list.size();
    }
}

@Override
public void onStop() {
    super.onStop();
    if (mAuthListener != null) {
        mAuth.removeAuthStateListener(mAuthListener);
    }
}
}