onValidate method
Implementation
@override
ValidationResult onValidate(FieldController<DateTime> controller) =>
ValidationResult(
key: GenericValidators.betweenDates.name,
message: message,
isValid: controller.value != null
? (controller.value!.isAfter(minDate) ||
controller.value!.isAtSameMomentAs(minDate)) &&
(controller.value!.isBefore(maxDate) ||
controller.value!.isAtSameMomentAs(maxDate))
: true,
);