dismiss method
隐藏弹窗
Implementation
Future<bool> dismiss() {
  try {
    if (_isShowing) {
      _isShowing = false;
      Navigator.pop(_context);
      return Future<bool>.value(true);
    } else {
      return Future<bool>.value(false);
    }
  } catch (err) {
    return Future<bool>.value(false);
  }
}