Flutter Failed assertion: boolean expression must not be null - json

Question

I am on Vipin Vijayan's tutorial on parsing JSON in ListView following Flutter's fetch data from the internet cookbook and getting the following error:
═══════════════════════════════════ Exception caught by widgets library
═══════════════════════════════════ The following assertion was thrown
building MyApp(dirty, state: _MyAppState#a2d98): Failed assertion:
boolean expression must not be null
The relevant error-causing widget was MyApp
package:le_moineau/main.dart:35 When the exception was thrown, this
was the stack
#0 _MyAppState.build package:le_moineau/main.dart:70
#1 StatefulElement.build package:flutter/…/widgets/framework.dart:4802
#2 ComponentElement.performRebuild package:flutter/…/widgets/framework.dart:4685
#3 StatefulElement.performRebuild package:flutter/…/widgets/framework.dart:4857
#4 Element.rebuild package:flutter/…/widgets/framework.dart:4379 ...
Here is the code to my Users.dart parsed using https://app.quicktype.io/
// To parse this JSON data, do
//
// final users = usersFromJson(jsonString);
import 'dart:convert';
List<User> usersFromJson(String str) =>
List<User>.from(json.decode(str).map((x) => User.fromJson(x)));
String usersToJson(List<User> data) =>
json.encode(List<dynamic>.from(data.map((x) => x.toJson())));
class User {
User({
this.userId,
this.id,
this.title,
this.completed,
});
int userId;
int id;
String title;
bool completed;
factory User.fromJson(Map<String, dynamic> json) => User(
userId: json["userId"],
id: json["id"],
title: json["title"],
completed: json["completed"],
);
Map<String, dynamic> toJson() => {
"userId": userId,
"id": id,
"title": title,
"completed": completed,
};
}
and here's my main.dart code:
import 'dart:async';
//import 'dart:convert';
import 'dart:io';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'others/Users.dart';
class MyHttpOverrides extends HttpOverrides {
#override
HttpClient createHttpClient(SecurityContext context) {
return super.createHttpClient(context)
..badCertificateCallback =
(X509Certificate cert, String host, int port) => true;
}
}
Future<List<User>> getUsers() async {
final response =
await http.get(Uri.https('jsonplaceholder.typicode.com', 'todos'));
//await http.get("https://192.168.1.4:8081/json/todos.json");
if (response.statusCode == 200) {
// If the server did return a 200 OK response,
// then parse the JSON.
final List<User> users = usersFromJson(response.body);
return users;
} else {
// If the server did not return a 200 OK response,
// then throw an exception.
throw Exception('Failed to load users');
}
}
void main() => runApp(MyApp());
class MyApp extends StatefulWidget {
MyApp({Key key}) : super(key: key);
#override
_MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
List<User> _users;
bool _loading;
#override
void initState() {
HttpOverrides.global = new MyHttpOverrides();
super.initState();
_loading = true;
getUsers().then((users) {
setState(() {
_users = users;
_loading = false;
});
});
}
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Fetch Data Example',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: Scaffold(
appBar: AppBar(
title: Text(_loading ? 'Loading...' : 'Users'),
),
body: Container(
color: Colors.white,
child: ListView.builder(
itemCount: null == _users ? 0 : _users.length,
itemBuilder: (context, index) {
User user = _users[index];
return ListTile(
title: Text('This is a test'),
subtitle: Text(user.title),
);
}),
),
),
);
}
}
I get the same error even when I use Vipin's stock Widget Build code. Could someone take a look and give me some advice please?

Answer 1

try to initialize true value in bool variable
bool _loading = true;
and remove _loading = true; in initState()

Answer 2

Because Null Safety coming with Flutter 2.0.x, bool _loading; is not permitted, so you do either bool? _loading; or bool _loading = true;. ? means your variable accepts null-values.
UPDATE: Inserting code
NetService
class NetService {
...
static Future<T?> getJson<T>(String url, {int okCode = 200}) {
return http.get(Uri.parse(url))
.then((response) {
if (response.statusCode == okCode) {
return jsonDecode(response.body) as T;
}
_printDataNotOK(response);
return null;
})
.catchError((err) => _printError(err));
}
....
}
json_placeholder_service.dart
import 'dart:async';
import 'package:jsonplaceholder/services/networking.dart';
class JsonPlaceholderService {
static const _baseUrl = 'https://jsonplaceholder.typicode.com';
/* ---------------------------------------------------------------------------- */
static Future<List?> _doGet(String path) async {
return await NetService.getJson<List>(_baseUrl + path)
.whenComplete(() => print('\nFetching done!'));
}
/* ---------------------------------------------------------------------------- */
static Future<List?> fetchTodos() => _doGet('/todos');
}
home_page.dart
import 'package:flutter/material.dart';
import 'package:jsonplaceholder/services/json_placeholder_service.dart';
class HomePage extends StatelessWidget {
/* ---------------------------------------------------------------------------- */
const HomePage({Key? key}) : super(key: key);
/* ---------------------------------------------------------------------------- */
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Hi!'),
centerTitle: true,
),
body: SingleChildScrollView(
child: FutureBuilder<List?>(
future: JsonPlaceholderService.fetchTodos(),
builder: (context, snapshot) {
if (snapshot.hasError) return Text(snapshot.error.toString());
if (snapshot.connectionState == ConnectionState.done) {
var data = snapshot.data;
if (data != null && data.isNotEmpty) {
return Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: List<Widget>.generate(data.length, (i) {
var item = data[i];
return Text('${item['userId']} - ${item['id']} - ${item['title']}');
}),
);
}
return Text('No data received!');
}
return CircularProgressIndicator();
},
),
),
);
}
}
Result: