Error Adapter

private FirebaseListAdapter<Chat> adapter;
    RelativeLayout activity_main;

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if (item.getItemId() == R.id.signout){
            AuthUI.getInstance().signOut(this).addOnCompleteListener(new OnCompleteListener<Void>() {
                @Override
                public void onComplete(@NonNull Task<Void> task) {
                    Snackbar.make(activity_main, "Berhasil Logout!",Snackbar.LENGTH_SHORT).show();
                    finish();
                }
            });
        }
        return true;
    }

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

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

        activity_main = (RelativeLayout) findViewById(R.id.activity_main);
        findViewById(R.id.submit_button).setOnClickListener(this);
        displayChatMessage();
    }

    private void displayChatMessage() {
        ListView listOfMessage = (ListView)findViewById(R.id.list_of_message);
        adapter = new FirebaseListAdapter<Chat>(MainActivity.this,Chat.class,R.layout.chat,FirebaseDatabase.getInstance().getReference())
        {
            @Override
            protected void populateView(View v, Chat model, int position) {

                //Get references to the views of list_item.xml
                TextView messageText, messageUser, messageTime;
                messageText = (TextView) v.findViewById(R.id.messageText);
                messageUser = (TextView) v.findViewById(R.id.messageUser);
                messageTime = (TextView) v.findViewById(R.id.messageTime);

                messageText.setText(model.getMessageText());
                messageUser.setText(model.getMessageUser());
                messageTime.setText(DateFormat.format("dd-MM-yyyy (HH:mm:ss)", model.getMessageTime()));

            }
        };
        listOfMessage.setAdapter(adapter);
    }

    @Override
    public void onClick(View view) {
        int i = view.getId();
        if (i == R.id.submit_button){
            EditText input = (EditText) findViewById(R.id.input);
            FirebaseDatabase.getInstance().getReference().push().setValue(new Chat(input.getText().toString(),FirebaseAuth.getInstance().getCurrentUser().getEmail()));
            input.setText("");
        }
    }