Type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast error

Question

In my sample project I implemented project with riverpod package and freezed. In the code below, I can get data from the server successfully, but when I try to use model.fromJson I get this error:

getting type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast

Server response data:

{
  "message": "test message",
  "statusCode": 1
}

login model to cast data and structure of server response:

LoginModel loginModelFromJson(Map<String, dynamic> str) => LoginModel.fromJson(str);

String loginModelToJson(LoginModel data) => json.encode(data.toJson());

@freezed
class LoginModel with _$LoginModel {
  const factory LoginModel({
    required LoginResponse response,
  }) = _LoginModel;

  factory LoginModel.fromJson(Map<String, dynamic> json) => _$LoginModelFromJson(json);
}

@freezed
class LoginResponse with _$LoginResponse {
  const factory LoginResponse({
    required String message,
    required int statusCode,
  }) = _LoginResponse;

  factory LoginResponse.fromJson(Map<String, dynamic> json) => _$LoginResponseFromJson(json);
}

Here on LoginResponse class I defined two parameters message and statusCode which are returned by server. When I try to use this request such as:

Future<LoginModel> getResponse(String mobileNumber) async {
  const endPoint = 'http://192.168.1.11/api';
  try {
    final response = await _read(dioProvider).get('${endPoint}/register');

    /*GETTING ERROR HERE*/
    return loginModelFromJson(response.data as Map<String, dynamic>);
  } on DioError catch (e) {
    ///
  }
}

I get the previously mentioned error on this line of getResponse method:

return loginModelFromJson(response.data as Map<String, dynamic>);

Shiri

Answer 1

Your response JSON is empty or contains incorrect content. That is why Dart is not able to cast it as Map<String, dynamic>. You have to do it manually

LoginResponse _$LoginResponseFromJson(Map<String, dynamic> json) {
  return LoginResponse(
    message: json['message'] as String,
    statusCode: json['statusCode'] as int,
  );
}

CHANGE

return loginModelFromJson(response.data as Map<String, dynamic>);

TO

return LoginResponse.fromJson(response.data);

Opiate

i should edit _$LoginResponseFromJson in generated freezed class? – Shiri

If you are using json_serialize, you should use return LoginResponse.fromJson(response.data); – Opiate

i used freezed not json_serialize – Shiri

It looks same usage for me as json_serialize – Opiate

is this code correct? @freezed class LoginResponse with _$LoginResponse { const factory LoginResponse({ required String message, required int statusCode, }) = _LoginResponse; factory LoginResponse.fromJson(Map<String, dynamic> json) => _$LoginResponseFromJson(json); } could you help me how can i resolve that please? – Shiri

I am not using freezed. If this code is autogenerated it is probably correct. Your usage of it is not. In Dart/Flutter you can't just cast directy to specific type. Dart is not that smart in most cases. See the edit of my answer. – Opiate

Let us continue this discussion in chat. – Shiri

for your updated code i get this error: A value of type 'LoginResponse' can't be returned from the method 'getResponse' because it has a return type of 'Future<LoginModel>'. – Shiri

Answer 2

you are getting a null response. Try this code and see if the error still there or not.

if(response.data!=null)
{return loginModelFromJson(response.data as Map<String, dynamic>);}

Palmer

Answer 3

Use the json.decode() to obtain the Map<String, dynamic> object from json string.

Update the line return loginModelFromJson(response.data as Map<String, dynamic>); to return loginModelFromJson(json.decode(response.data));

Patentor

i get this error: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'String' – Shiri

Could you put these 3 lines before the return statement and check the console output? Need to see the exact value of the response.data 'print(response.data); Map<String, dynamic> parsed = json.decode(response.data); print(parsed);' – Patentor

Answer 4

In my case, the solution that helped me is null-safety!

This error can be handled by Dart null-safety. Using all variables in the Model class with null-safety is a good practice.

Example for you can:

CHANGE below

required String message,
required int statusCode,

WITH

required String? message,
required int? statusCode,

And while using the variable you can use like: Text(message ?? ''),

Georama