Исключение нулевого указателя студии Android на findViewById-int

Ошибка, которую я получаю:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View com.github.irshulx.Editor.findViewById(int)' on a null object reference at com.example.verma.inspire.PostActivity.onCreate(PostActivity.java:45) at android.app.Activity.performCreate(Activity.java:6975) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)*

файл PostActivity.java:

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.view.LayoutInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;

import com.github.irshulx.models.EditorTextStyle;
import com.github.irshulx.Editor;

public class PostActivity extends AppCompatActivity {

    private Toolbar mToolbar;
    private Editor editor;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_post);

        mToolbar = (Toolbar) findViewById(R.id.update_post_page_toolbar);
        setSupportActionBar(mToolbar);
        getSupportActionBar().setDisplayShowHomeEnabled(true);
        getSupportActionBar().setTitle("Update Post");

        editor = (Editor) editor.findViewById(R.id.editor); //error here
        setUpEditor();
    }

    ...
}

и макет XML:

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

    <include
        android:id = "@+id/update_post_page_toolbar"
        layout = "@layout/app_bar_layout" />

    <com.github.irshulx.Editor
        android:layout_width = "match_parent"
        android:id = "@+id/editor"
        app:render_type = "Editor"
        app:placeholder = "Start writing here..."
        android:paddingTop = "10dp"
        android:paddingLeft = "10dp"
        android:paddingRight = "10dp"
        android:layout_height = "wrap_content"
        android:paddingBottom = "100dp" />

</RelativeLayout>