validate method

Implementation

@override
String? validate(DropdownOption? data) {
  print(data);
  print('sjkbhjfdb');
  print(data != null);
  print(data?.label);
  print(data?.id);
  if (data == null) {
    print('jfsbhjbdfbdf');
    return 'No option selected';
  }
  return null;
}