How to catch exception in flutter?

Try

void loginUser(String email, String password) async {
  try {
    var user = await _data
      .userLogin(email, password);
    _view.onLoginComplete(user);
      });
  } on FetchDataException catch(e) {
    print('error caught: $e');
    _view.onLoginError();
  }
}

catchError is sometimes a bit tricky to get right.
With async/await you can use try/catch like with sync code and it is usually much easier to get right.