Screen not loading after login is authenticated in flutter - flutter

Question

I have created a route to the home screen which should be displayed after login is successful. However, my circular bar just keeps moving (indicating isLoading stage) and the home screen does not show up in the simulator.
I am aware that the login is successful since my response variable does print the information in the console that it accesses after login.
On pressing the login the button 2 main issues are displayed:
1) Exception thrown while handling a gesture
2) Unhandled Exception: Failed assertion: boolean expression must not be null
I am unable to solve this these 2 issues. Any help would be great.
Thanks in advance!
There is no compilation error. The following are the main issues shown after pressing the login button (complete stack tree and message posted later in the question):
flutter: The following ArgumentError was thrown while handling a gesture:
flutter: Invalid argument (onError): Error handler must accept one Object or one Object and a StackTrace as
flutter: arguments, and return a a valid result: Closure: (Exception) => void
Unhandled Exception: Failed assertion: boolean expression must not be null
The console message posted later refers to 3 files. I have posted important code of the files below.
File: login_screen.dart
import 'dart:ui';
import 'package:flutter/material.dart';
class LoginScreen extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return new LoginScreenState();
}
}
class LoginScreenState extends State<LoginScreen>
implements LoginScreenContract, AuthStateListener {
BuildContext _ctx;
bool _isLoading = false;
final formKey = new GlobalKey<FormState>();
final scaffoldKey = new GlobalKey<ScaffoldState>();
String _password, _username;
LoginScreenPresenter _presenter;
LoginScreenState() {
_presenter = new LoginScreenPresenter(this);
var authStateProvider = new AuthStateProvider();
authStateProvider.subscribe(this);
}
void _submit() {
final form = formKey.currentState;
if (form.validate()) {
setState(() => _isLoading = true);
form.save();
_presenter.doLogin(_username, _password);
}
}
void _showSnackBar(String text) {
scaffoldKey.currentState
.showSnackBar(new SnackBar(content: new Text(text)));
}
#override
onAuthStateChanged(AuthState state) {
if(state == AuthState.LOGGED_IN)
Navigator.of(_ctx).pushReplacementNamed("/home");
}
#override
Widget build(BuildContext context) {
_ctx = context;
....(UI for login screen)
#override
void onLoginError(String errorTxt) {
_showSnackBar(errorTxt);
setState(() => _isLoading = false);
}
#override
void onLoginSuccess(User user) async {
HomeScreen()));
_showSnackBar(user.toString());
setState(() => _isLoading = false);
var db = new DatabaseHelper();
await db.saveUser(user);
var authStateProvider = new AuthStateProvider();
HomeScreen()));
authStateProvider.notify(AuthState.LOGGED_IN);
onAuthStateChanged(AuthState.LOGGED_IN);
}
}
File: login_screen_presenter.dart
import 'package:better_login/rest_ds.dart';
import 'package:better_login/user.dart';
abstract class LoginScreenContract {
void onLoginSuccess(User user);
void onLoginError(String errorTxt);
}
class LoginScreenPresenter {
LoginScreenContract _view;
RestDatasource api = new RestDatasource();
LoginScreenPresenter(this._view);
doLogin(String username, String password) {
api.login(username, password).then((User user) {
_view.onLoginSuccess(user);
}).catchError((Exception error) =>
_view.onLoginError(error.toString()));
}
}
File: rest_ds.dart
static final LOGIN_URL = "https://legacy- api.example.com/v1/auth/login";
Future<User> login(String username, String password) {
return _netUtil.post(LOGIN_URL, body: {
"username": username,
"password": password
}).then((dynamic res) {
print(res.toString());
if(res["error"]){ throw new Exception(res["error_msg"]);}
return new User.map(res["user"]);
});
}
}
This is the complete info displayed in the console:
flutter: The following ArgumentError was thrown while handling a gesture:
flutter: Invalid argument (onError): Error handler must accept one Object or one Object and a StackTrace as
flutter: arguments, and return a a valid result: Closure: (Exception) => void
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #2 LoginScreenPresenter.doLogin (package:better_login/login_screen_presenter.dart:17:8)
flutter: #3 LoginScreenState._submit (package:better_login/login_screen.dart:41:18)
flutter: #4 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:511:14)
flutter: #5 _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:566:30)
flutter: #6 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:166:24)
flutter: #7 TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:240:9)
flutter: #8 TapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:177:9)
flutter: #9 PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:436:9)
flutter: #10 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:73:12)
flutter: #11 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:101:11)
flutter: #12 _WidgetsFlutterBinding&BindingBase&GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:221:19)
flutter: #13 _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:199:22)
flutter: #14 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
flutter: #15 _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
flutter: #16 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
flutter: #20 _invoke1 (dart:ui/hooks.dart:233:10)
flutter: #21 _dispatchPointerDataPacket (dart:ui/hooks.dart:154:5)
flutter: (elided 5 frames from package dart:async)
flutter:
flutter: Handler: onTap
flutter: Recognizer:
flutter: TapGestureRecognizer#398b8(debugOwner: GestureDetector, state: possible, won arena, finalPosition:
flutter: Offset(194.0, 504.0), sent tap down)
flutter: ════════════════════════════════════════════════════════════════════════════════════════════════════
flutter: {status: success, message: Authentication Success, executionTime: 0.052, data: {token: U2FsdGVkX19ZQS5wQXRYWTkh2o4PyrtmhS4kELJO0WsEBDbn30G9Oig/13fzHzqZ, custKey: anb2mNXFERnJ4IQW....(rest is info got on login)
[VERBOSE-2:ui_dart_state.cc(148)] Unhandled Exception: Failed assertion: boolean expression must not be null
#0 RestDatasource.login.<anonymous closure> (package:better_login/rest_ds.dart:21:13)
#1 _rootRunUnary (dart:async/zone.dart:1132:38)
#2 _CustomZone.runUnary (dart:async/zone.dart:1029:19)
#3 _FutureListener.handleValue (dart:async/future_impl.dart:126:18)
#4 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:639:45)
#5 Future._propagateToListeners (dart:async/future_impl.dart:668:32)
#6 Future._complete (dart:async/future_impl.dart:473:7)
#7 _SyncCompleter.complete (dart:async/future_impl.dart:51:12)
#8 _AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:28:18)
#9 _completeOnAsyncReturn (dart:async-patch/async_patch.dart:294:13)
#10 _withClient (package:http/http.dart)
<asynchronous suspension>
#11 post (package:http/http.dart:69:5)
#12 NetworkUtil.post (package:better_login/network_util.dart<…>

Answer 1

There are several issues in your code:
catchError param is not of type Exception is of type Object. You can use the second param of catchError, test to filter errors received. This is the cause of the crash report trace The following ArgumentError was thrown while handling a gestur.
the main cause of the crash is here if(res["error"]){ throw new Exception(res["error_msg"]);}, because the res["error"] is probably null. To fix it change it to if(res["error"] != null) .....
Refactoring points:
you don't need to save the build context in a _ctx variable, any subclass of State has access to it's main context via context property.