An exception "flutter: type 'Null' is not a subtype of type 'String'" is thrown when formatting json data in flutter

I want to fetch and format json data from this as a trial in flutter. However, during the formatting process, an exception occurs: type 'Null' is not a subtype of type 'String'.

And these are my code:

user_model.dart

class User {
  int    id;
  String email;
  String firstName;
  String lastName;
  String avator;
  User({
    required this.id,
    required this.email,
    required this.firstName,
    required this.lastName,
    required this.avator
  });

  factory User.fromJson(Map<String, dynamic> json) => User(
    id        : json['id'],
    email     : json['email'],
    firstName : json['first_name'],
    lastName  : json['last_name'],
    avator    : json['avator']
  );
}

user_api.dart

...
class UserApi {
  Future<List<User>?> getUsers() async {
    final url = Uri.parse('https://reqres.in/api/users?page=2');
    try {
      final res = await http.get(url);
      if (res.statusCode == 200) {
        final Map<String, dynamic> body  = jsonDecode(res.body);
        final List<User> users = body['data'].map((dynamic userData) => {
          print('userData : $userData');
          User.fromJson(userData) // There seems to be an error here.
        }).toList();
        return users;
      } else {
        return null;
      }
    } catch (e) {
      print(e.toString());
    }
    return null;
  }
}

And userData seems like this in my console:

flutter: userData : {id: 7, email: [email protected], first_name: Michael, last_name: Lawson, avatar: https://reqres.in/img/faces/7-image.jpg}

I don’t think userData is kind of Null, but why do I get the exception?