toJson method Null safety
override
    Implementation
@override
Map<String, dynamic> toJson() {
  final json = super.toJson();
  json['type'] = type;
  json.addAll({
    'valueAsString': valueAsString,
  });
  return json;
}@override
Map<String, dynamic> toJson() {
  final json = super.toJson();
  json['type'] = type;
  json.addAll({
    'valueAsString': valueAsString,
  });
  return json;
}