由于对话框，我的应用程序崩溃

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

public void onBackPressed() {
    AlertDialog.Builder builder = new AlertDialog.Builder
            (getApplicationContext(), android.R.style.Theme_DeviceDefault);
    builder.setTitle("Exit").setMessage("Do you really want to Exit ? ")
    .setPositiveButton(" Yes", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialogInterface, int i) {
                    System.exit(1);
                }
            })
    .setNegativeButton("No", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialogInterface, int i) {
            dialogInterface.dismiss();
        }
    });
    AlertDialog dialog = builder.create();
    dialog.show();
}

当我按下后退按钮时，此应用程序崩溃。