MainActivity of chat (db not working)

package com.example.otherchat;

import android.support.annotation.NonNull;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.format.DateFormat;
import android.view.View;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import com.firebase.ui.auth.AuthUI;
import com.firebase.ui.database.FirebaseListAdapter;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.FirebaseApp;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.database.FirebaseDatabase;
import android.content.Intent;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity {

    final int SIGN_IN_REQUEST_CODE = 10;
    private FirebaseListAdapter<ChatMessage> adapter;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        FirebaseApp.initializeApp(getApplicationContext());
        setContentView(R.layout.activity_main);

        if(FirebaseAuth.getInstance().getCurrentUser() == null) {
            // Start sign in/sign up activity
            startActivityForResult(
                    AuthUI.getInstance()
                            .createSignInIntentBuilder()
                            .build(),
                    SIGN_IN_REQUEST_CODE
            );
        } else {
            // User is already signed in. Therefore, display
            // a welcome Toast
            Toast.makeText(this,
                    "Welcome " + FirebaseAuth.getInstance()
                            .getCurrentUser()
                            .getDisplayName(),
                    Toast.LENGTH_LONG)
                    .show();

            // Load chat room contents
            displayChatMessages();

            FloatingActionButton fab =
                    (FloatingActionButton)findViewById(R.id.fab);

            fab.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    EditText input = (EditText)findViewById(R.id.input);

                    // Read the input field and push a new instance
                    // of ChatMessage to the Firebase database
                    FirebaseDatabase.getInstance()
                            .getReference()
                            .push()
                            .setValue(new ChatMessage(input.getText().toString(),
                                    FirebaseAuth.getInstance()
                                            .getCurrentUser()
                                            .getDisplayName())
                            );

                    // Clear the input
                    input.setText("");
                }
            });
        }
    }

    private void displayChatMessages()
    {
        ListView listOfMessages = (ListView)findViewById(R.id.list_of_messages);

        adapter = new FirebaseListAdapter<ChatMessage>(this, ChatMessage.class,
                R.layout.message, FirebaseDatabase.getInstance().getReference()) {
            @Override
            protected void populateView(View v, ChatMessage model, int position) {
                // Get references to the views of message.xml
                TextView messageText = (TextView)v.findViewById(R.id.message_text);
                TextView messageUser = (TextView)v.findViewById(R.id.message_user);
                TextView messageTime = (TextView)v.findViewById(R.id.message_time);

                // Set their text
                messageText.setText(model.getMessageText());
                messageUser.setText(model.getMessageUser());

                // Format the date before showing it
                messageTime.setText(DateFormat.format("dd-MM-yyyy (HH:mm:ss)",
                        model.getMessageTime()));
            }
        };

        listOfMessages.setAdapter(adapter);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode,
                                    Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if(requestCode == SIGN_IN_REQUEST_CODE) {
            if(resultCode == RESULT_OK) {
                Toast.makeText(this,
                        "Successfully signed in. Welcome!",
                        Toast.LENGTH_LONG)
                        .show();
                displayChatMessages();
            } else {
                Toast.makeText(this,
                        "We couldn't sign you in. Please try again later.",
                        Toast.LENGTH_LONG)
                        .show();

                // Close the app
                finish();
            }
        }

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main_menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if(item.getItemId() == R.id.menu_sign_out) {
            AuthUI.getInstance().signOut(this)
                    .addOnCompleteListener(new OnCompleteListener<Void>() {
                        @Override
                        public void onComplete(@NonNull Task<Void> task) {
                            Toast.makeText(MainActivity.this,
                                    "You have been signed out.",
                                    Toast.LENGTH_LONG)
                                    .show();

                            // Close activity
                            finish();
                        }
                    });
        }
        return true;
    }
}