type 'Null' is not a subtype of type 'String' when get data

I have a response in JSON:

{
    "data": {
        "id": "12-43-abc",
        "number": "4",
        "name": "Admin Test 1",
        "stage": "active",
        "sex": "female",
        "title": "admin",
        "dob": null,
        "phoneNumber": "1",
        "email": "admin1@gmail.com",
        "address": null,
        "mId": null,
        "createdDateTime": "2020-09-17T02:42:10.447Z",
        "totalRemain": null,
        "carryOverRemain": null,
        "fromTime": null,
        "toTime": null
    }
}

To convert JSON to Object, I create a model:

class User {
  String? id;
  String? number;
  String? name;
  String? stage;
  String? sex;
  String? title;
  DateTime? dob;
  String? phoneNumber;
  String? email;
  String? address;
  String? mId;
  DateTime? createdDateTime;
  int? totalRemain;
  int? carryOverRemain;
  DateTime? fromTime;
  DateTime? toTime;

  User({
      this.id,
      this.number,
      this.name,
      this.stage,
      this.sex,
      this.title,
      this.dob,
      this.phoneNumber,
      this.email,
      this.address,
      this.mId,
      this.createdDateTime,
      this.totalRemain,
      this.carryOverRemain,
      this.fromTime,
      this.toTime});

  factory User.fromJson(Map<String, dynamic> json) {
    return User(
      id: json['id'],
      number: json['badgeNumber'] ?? "null",
      name: json['name'] ?? "null",
      stage: json['status'],
      sex: json['gender'] ?? "null",
      title: json['title'] ?? "null",
      dob: DateTime.parse(json['birthDay']),
      phoneNumber: json['phone'] ?? "null",
      email: json['email'] ?? "null",
      address: json['address'] ?? "null",
      mId: json['managerId'] ?? "null",
      createdDateTime: DateTime.parse(json['createdDateTime']),
      totalRemain: json['totalRemain'],
      carryOverRemain: json['carryOverRemain'],
      fromTime: DateTime.parse(json['fromTime']),
      toTime: DateTime.parse(json['toTime'])
    );
  }
}

I create a signIn method:

Future<Either<Failure, dynamic>> signIn(String number, String password) async {
    try {
      Map<String, String> bd = {
        'number': number,
        'password': password
      };

      final Response res = await dio.post(Endpoint.signIn, data: bd);

      if(res.statusCode == 200) {
        return Right(User.fromJson(res.data['data']));
      }
      else {
        return Right(res.data['error']);
      }
    }
    catch(e) {
      throw e;
      return Left(SystemFailure());
    }
  }

When debugging, the status code is 200 but it throws an exception type ‘Null’ is not a subtype of type ‘String’. I’ve searched Google for it but still have no answer. No idea what’s wrong in my model class. Thanks for your help.

>Solution :

The problem is with dob, fromTime and toTime because they are null. You can’t do DateTime.parse() on null. So maybe you can replace this

toTime: DateTime.parse(json['toTime'])

with

toTime: json['toTime'] != null ? DateTime.parse(json['toTime']) : null

for example. The same logic with dob and toTime. Probably also with createdDateTime in case that one happens to be null as well