package ru.mail.park.chat.activities;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class DialogActivity extends AppCompatActivity {

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

package ru.mail.park.chat.activities;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import ru.mail.park.chat.R;

public class DialogActivity extends AppCompatActivity {

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

fix R in dialog activity