The setter 'msg=' was called on null - dart

Question

I am sending a request to an api with flutter and saving the response values in instance of one of my models. but i am getting some strange errors. you can see my code and errors that i am getting below. I am using scoped_model for state management.
import 'dart:convert';
import 'dart:async';
import 'dart:io';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:scoped_model/scoped_model.dart';
import 'package:http/http.dart' as http;
import 'package:rxdart/subjects.dart';
class Patient {
final String guid;
final String phone;
final String token;
final String name;
final String lastName;
Patient({#required this.guid, #required this.phone, #required this.token , #required this.name , #required this.lastName });
}
class RTN {
String msg = '';
String success = 'false';
String guid ='';
String srcPath ='';
String thumbPath ='';
dynamic obj ='';
RTN(
{#required this.msg,
#required this.guid,
#required this.obj,
#required this.srcPath,
#required this.success,
#required this.thumbPath});
}
class MainModel extends Model with Connected, PatientModel, UtilityModel {
}
mixin Connected on Model {
List<Doctor> _doctor = [];
Patient _authenticatedPatient;
bool _isLoading = false;
RTN _rtn;
RTN get rtn{
return _rtn;
}
List<Doctor> get allDrs {
return List.from(_doctor);
}
}
mixin PatientModel on Connected {
PublishSubject<bool> _patientSubject = PublishSubject();
Patient get patient {
return _authenticatedPatient;
}
PublishSubject<bool> get patientSubject {
return _patientSubject;
}
Future<Map<String, dynamic>> authenticateFirst(String phone) async {
try{
_isLoading = true;
notifyListeners();
final Map<String, dynamic> authFirstData = {
'Phone': phone,
};
http.Response response;
response = await http.post(
'http://guffy.ir/Auth/RegisterOne',
body: json.encode(authFirstData),
headers: {'Content-Type': 'application/json'},
);
final Map<String, dynamic> responseData = json.decode(response.body);
print(responseData);
bool hasError = true;
String message = '';
print(responseData);
if (responseData.containsKey("guid")) {
hasError = false;
rtn.msg = responseData["msg"];
rtn.guid = responseData["guid"];
rtn.obj = responseData["obj"];
rtn.srcPath = responseData["srcPath"];
rtn.success = responseData["success"];
rtn.thumbPath = responseData["thumbPath"];
message = rtn.msg;
_authenticatedPatient = Patient(
guid: responseData["guid"],
phone: phone,
token: null,
name: null,
lastName: null,
);
// _patientSubject.add(true);
final SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setString('token', '');
prefs.setString('patientPhone', phone);
prefs.setString('patientGuid', responseData['guid']);
prefs.setString('name', '');
prefs.setString('lastName', '');
}
_isLoading = false;
notifyListeners();
return {'success': !hasError, 'message': message};
}
catch(error){
_isLoading = false;
print(error);
}}
Future<Map<String, dynamic>> authenticateSecond(String phone) async {
_isLoading = true;
notifyListeners();
final Map<String, dynamic> authFirstData = {
'Phone': phone,
};
http.Response response;
response = await http.post(
'http://guffy.ir/Auth/RegisterOne',
body: json.encode(authFirstData),
headers: {'Content-Type': 'application/json'},
);
final Map<String, dynamic> responseData = json.decode(response.body);
print(responseData);
bool hasError = true;
String message = '';
print(responseData);
if (responseData.containsKey('guid')) {
hasError = false;
message = responseData['msg'];
_authenticatedPatient = Patient(
guid: responseData['guid'],
phone: phone,
token: null,
name: null,
lastName: null,
);
// _patientSubject.add(true);
final SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setString('token', null);
prefs.setString('patientPhone', phone);
prefs.setString('patientGuid', responseData['guid']);
prefs.setString('name', null);
prefs.setString('lastName', null);
}
_isLoading = false;
notifyListeners();
return {'success': !hasError, 'message': message};
}
Future<Map<String, dynamic>> authenticateThird(String phone) async {
_isLoading = true;
notifyListeners();
final Map<String, dynamic> authThirdData = {
'Phone': phone,
};
http.Response response;
response = await http.post(
'http://guffy.ir/Auth/RegisterOne',
body: json.encode(authThirdData),
headers: {'Content-Type': 'application/json'},
);
final Map<String, dynamic> responseData = json.decode(response.body);
print(responseData);
bool hasError = true;
String message = '';
print(responseData);
if (responseData.containsKey('guid')) {
hasError = false;
message = responseData['msg'];
_authenticatedPatient = Patient(
guid: responseData['guid'],
phone: phone,
token: null,
name: null,
lastName: null,
);
_patientSubject.add(true);
final SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setString('token', null);
prefs.setString('patientPhone', phone);
prefs.setString('patientGuid', responseData['guid']);
prefs.setString('name', null);
prefs.setString('lastName', null);
}
_isLoading = false;
notifyListeners();
return {'success': !hasError, 'message': message};
}
void autoAuthenticate() async {
final SharedPreferences prefs = await SharedPreferences.getInstance();
final String name = prefs.getString('name');
final String lastName = prefs.getString('lastName');
if (name != '' && lastName != '') {
final String patientPhone = prefs.getString('patientPhone');
final String patientGuid = prefs.getString('patientGuid');
final String token = prefs.getString('token');
_authenticatedPatient =
Patient(guid: patientGuid, phone: patientPhone, token: token , name: name , lastName: lastName);
_patientSubject.add(true);
notifyListeners();
}
}
void logout() async {
_authenticatedPatient = null;
_patientSubject.add(false);
final SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.remove('token');
prefs.remove('PatientEmail');
prefs.remove('PatientId');
}
}
mixin UtilityModel on Connected {
bool get isLoading {
return _isLoading;
}
}
this is the response that i am getting from the api:
{msg: کد ارسال شد, success: true, guid: 15dde915-d752-4bf1-90ad-49872868af35, srcPath: null, thumbPath: null, obj: null}
this is the error that i am getting:
I/flutter (10151): NoSuchMethodError: The setter 'msg=' was called on null.
I/flutter (10151): Receiver: null
I/flutter (10151): Tried calling: msg="کد ارسال شد"
E/flutter (10151): [ERROR:flutter/shell/common/shell.cc(184)] Dart Error: Unhandled exception:
E/flutter (10151): NoSuchMethodError: The method '[]' was called on null.
E/flutter (10151): Receiver: null
E/flutter (10151): Tried calling: []("success")
E/flutter (10151): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
E/flutter (10151): #1 _AuthOnePageState._submitForm (package:restt/pages/authone.dart:93:27)
E/flutter (10151): <asynchronous suspension>

Answer 1

The error is saying that you tried to access the field msg on a null object, which likely is this line:
rtn.msg = responseData["msg"];
You just need to instantiate the object before using it:
RTN _rtx;
RTN get rtn {
if (_rtn == null) {
_rtx = RTN(); // Instantiate the object if its null.
}
return _rtn;
}

Answer 2

Initializing the class is enough
RTN _rtx = RTN();