Dart inheritance issue

main.dart

// Model interface
abstract class JsonConvertible {
  Map<String, dynamic> toJson();
  JsonConvertible.fromJson(Map<String, dynamic> json);
}

// Model
class AccessTokenResponse extends JsonConvertible {
  String? accessToken;

  @override
  Map<String, dynamic> toJson() {
    return {};
  }

  @override
  AccessTokenResponse.fromJson(Map<String, dynamic> json)
      : super.fromJson(json) {
    accessToken = json['access_token'];
  }
}

// Network response class
class NetworkResponse<Model> {
  Model data;
  
  NetworkResponse.ok(this.data);
}

// Class to create a valid network service request
class NetworkRequest {
}

// Class that performs all network calls
class NetworkService {
  Future<NetworkResponse<M>> execute<M extends JsonConvertible>(NetworkRequest request) async {
    // For simplicity replaced all DIO calls with static data:
    final response = {'data': {'access_token': 'XXX'}};

    return NetworkResponse.ok(M.fromJson(response['data'])); //<- Fails here with error: Method 'fromJson' isn't defined for the type 'Type'...
  }
}

 

void main() {
  for (int i = 0; i < 5; i++) {
    print('hello ${i + 1}');
  }
}