code: @Override
      public void onCancel(DialogInterface dialog) {
          super.onCancel(dialog);
  
          if (mListener == null) {
              throw new NullPointerException(
                      "Listener no longer exists in onCancel()");
          }
  
          mListener.onDateTimeCancel();
      }
docs: <p>Called when the user clicks outside the dialog or presses the <b>Back</b>
  button.</p>
  <p>
  <p><b>Note:</b> Actual <b>Cancel</b> button clicks are handled by {@code mCancelButton}'s
  event handler.</p>