Android onKeyDown вернуться к основной деятельности

Я новичок в Android. Я хотел бы разработать функцию из фрагмента обратно на домашний экран для выхода из приложения, но она не работает. В настоящее время, как только я нажал кнопку «Назад», будет предложено выйти из приложения. Как вернуться к основному действию, чтобы выйти из приложения из onKeyDown?

 public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener, FragmentFlickrGridImage.OnFragmentInteractionListener
     {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    if (savedInstanceState == null) {
        Fragment fragment = null;
        Class fragmentClass = null;
        fragment = new MyFragment("0");
        try {
            fragment = (Fragment) fragmentClass.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }

        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
    }


    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setItemIconTintList(null);
    navigationView.setNavigationItemSelectedListener(this);
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.share) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();
    Fragment fragment = null;
    Class fragmentClass = null;
    String title="";

    if (id == R.id.one) {
        fragment = new MyFragment("1");
        title="One";
    } else if (id == R.id.two) {
        fragment = new MyFragment("2");
        title ="two";
    } else if (id == R.id.three) {
        fragment = new MyFragment("3");
        title="three";
    }

    try {
        fragment = (Fragment) fragmentClass.newInstance();
    } catch (Exception e) {
        e.printStackTrace();
    }
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
    getSupportActionBar().setTitle(title);
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

@Override
public void onFragmentInteraction(Uri uri) {

}
// confirm when user click back key
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode != KeyEvent.KEYCODE_BACK)  return super.onKeyDown(keyCode, event);

    DialogInterface.OnClickListener dialogClickListener = new DialogInterface.OnClickListener() {

        public void onClick(DialogInterface dialog, int which) {
            switch (which){
                case DialogInterface.BUTTON_POSITIVE:
                    //Yes button clicked
                    finish();
                    break;

                case DialogInterface.BUTTON_NEGATIVE:
                    //No button clicked
                    break;
            }
        }
    };

    AlertDialog.Builder builder = new AlertDialog.Builder(this)
            .setMessage("Are you sure?")
            .setPositiveButton("Yes", dialogClickListener)
            .setNegativeButton("No", dialogClickListener)
            .setCancelable(false)
            .setTitle("Exit");
    builder.show();

    return super.onKeyDown(keyCode, event);
} 

Ниже приведен код ошибки, который я получил при реализации addtobackstack.

Application is not responding: AppWindowToken{bcc0a65 token=Token{1a7f775c ActivityRecord{3ccf31cf u0 com.google.android.googlequicksearchbox/com.google.android.launcher.GEL t310}}}.  It has been 5010.0ms since event, 5010.0ms since wait started.  
Reason: Waiting because no window has focus but there is a focused application that may eventually add a window when it finishes starting up.