Firebase CurrentUser Null, как сохранить пользователя во всех действиях?

I'm making a blog app, the registration works great, when successful, a new intent where there's a userProfileActivity starts, I added in this activity a button to write a post (opens a new activity with editTexts to fill ), the problem is: the currentUser is null on all the activities except the registration one, does this have to do with calling the function finish() after opening a new activity? can you please explain to me how to keep the user logged in?

/*after successful registration,the profile activity starts, here's what inside the oncreate methode*/

fabAddClasseP.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent addPostIntent = new Intent(ProfileActivity.this, addPostActivity.class);

                startActivity(addPostIntent);


        }
    });

/* the addPostActivity*/
/* declaration*/
    private FirebaseAuth auth;
/*a reference to the post*/
    private DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference().child("dataPost");
/*a refernce to the user/author of post*/
    private DatabaseReference mDatabaseUser = FirebaseDatabase.getInstance().getReference().child("dataUser");

//then below on the onCreate methode, i make sure all edit texts are filled, 

buttonSavePost.setOnClickListener(new View.OnClickListener() {
//if  all filds are not empty
String PostId = mDatabase.child("PostId").push().getKey();



 //getting the current user id if not null
String userId = auth.getCurrentUser().getUid();





                       writeNewDataPost( userId, PostId, titlePost, contentPost);


    private void writeNewDataPost(String user_id,String post_id, String title, String content) {

        DataPost dataP = new DataPost(title,  content);

            mDatabase.child(Post_id).setValue(dataP);
/*adding a 

node to the current post "author, and setting its value to the current logged in user*/    
    mDatabase.child(post_id).child("author").setValue(user_id);

               }
        }