onValidate method
override
    onValidate is used to validate the given input by the user.
Implementation
@override
String? onValidate(
  int? value,
  String validationMessage,
) {
  if (mandatory) {}
  return null;
}onValidate is used to validate the given input by the user.
@override
String? onValidate(
  int? value,
  String validationMessage,
) {
  if (mandatory) {}
  return null;
}