getValidationMessage function
Returns the validation message for the given key
Implementation
String? getValidationMessage(String key) {
final validatorForKey = _PaymentFormTextValidations[key];
if (validatorForKey == null) return null;
String? validationMessageForKey = validatorForKey(
_PaymentFormTextEditingControllers[key]!.text,
);
return validationMessageForKey;
}