I keep getting an error in flutter, Uncaught (in promise) Error: Expected a value of type 'String', but got one of type 'TypeErrorImpl'

This happens after fetching data from an api.

The request executes successfully and i get a json response of departments but I can’t process the response. Uncaught (in promise) Error: Expected a value of type ‘String’, but got one of type ‘TypeErrorImpl’

The json response object

{
    "success": true,
    "data": [
        {
            "id": "56c8b5f8-7f74-4f0c-9fc0-9a3c411bc3a4",
            "image": "http://127.0.0.1:8000/media/api/uploads/departments/2023/03/14/child_yk4u2KO.png",
            "name": "Education And Child Development",
            "slug": "education-and-child-development",
            "created_at": "2023-02-23T14:51:39Z",
            "updated_at": "2023-03-14T14:24:10.699998Z"
        }
    ],
    "message": "Departments successfuly retrieved"
}

The department.dart file

class Department {
  String id;
  String image;
  String name;
  String slug;

  Department({
    this.id,
    this.image,
    this.name,
    this.slug,
  });

  Department.fromJson(Map<String, dynamic> json) {
    id = json['id'].toString();
    image = json['image'] != null ? json['image'] : null;
    name = json['name'] != null ? json['name'] : null;
    slug = json['slug'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['id'] = this.id;
    data['image'] = this.image;
    data['name'] = this.name;
    data['slug'] = this.slug;
    return data;
  }
}

The function fetching the data

Future<Stream<Department>> getSchoolDepartments() async {
  final String url =
      '${GlobalConfiguration().getString('api_base_url')}departments';

  final client = new http.Client();

  try {
    final streamedRest = await client.send(http.Request('get', Uri.parse(url)));

    print("Status code : " + streamedRest.statusCode.toString());

    return streamedRest.stream
        .transform(utf8.decoder)
        .transform(json.decoder)
        .map((data) => Helper.getData(data))
        .expand((data) => (data as List))
        .map((data) {
      print("data : " + data);
      return Department.fromJson(data);
    });
  } on Exception catch (e) {
    print(e);
  }
}

The processing function in school_controller.dart

  void listenForDepartments() async {
    print("starting departments stream");
    final Stream<Department> stream = await getSchoolDepartments();
    stream.listen((Department _department) {
      setState(() => departments.add(_department));
    }, onError: (a) {
      print("Failed to add department : " + a);
    }, onDone: () {});
  }

I am expecting a successfull processing with departments object populated with data from json response.