_TypeError(类型 'Null' 不是类型"字符串"的子类型)

Question

我不知道发生了什么，但我试图通过使其为空来解决这个问题，但它不起作用。我想从数据库中查看元素，因此我把它们放在"for"循环. .但它仍然显示我异常_TypeError(类型'Null'不是类型'String'的子类型)那么我该怎么做才能解决这个问题呢?下面是异常的截图:输入图片描述

这是我的代码:

`import 'pac`kage:blackboard/view/Teacher/Addcourse1.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:blackboard/constraints/textstyle.dart';
import 'package:flutter/material.dart';
import 'package:blackboard/setting/colors.dart';
class CoursesT extends StatefulWidget {
const CoursesT({Key? key}) : super(key: key);
@override
State<CoursesT> createState() => _CoursesTState();
}
class _CoursesTState extends State<CoursesT> {
// Getting Student all Records
final Stream<QuerySnapshot>? studentRecords =
FirebaseFirestore.instance.collection('CourseStudent').snapshots();
// For Deleting Users
CollectionReference? delUser =
FirebaseFirestore.instance.collection('CourseStudent');
Future<void> _delete(id) {
return delUser!
.doc(id)
.delete()
.then((value) => print('User Deleted'))
.catchError((_) => print('Something Error In Deleted User'));
}
@override
Widget build(BuildContext context) {
return StreamBuilder<QuerySnapshot>(
stream: studentRecords,
builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
if (snapshot.hasError) {
print('Something Wrong in HomePage');
}
if (snapshot.connectionState == ConnectionState.waiting) {
return const Center(
child: CircularProgressIndicator(),
);
}
// Storing Data
final List? firebaseData = [];
snapshot.data?.docs.map((DocumentSnapshot documentSnapshot) {
Map store = documentSnapshot.data() as Map<String, dynamic>;
firebaseData!.add(store);
store['id'] = documentSnapshot.id;
}).toList();
return Scaffold(
appBar: AppBar(
backgroundColor: BBColors.primary6,
title: Text("Your Courses"),
leading: Icon(Icons.menu, color: Colors.white),
actions: [
Icon(
Icons.search,
),
SizedBox(
width: 20,
),
],
),
body: Container(
margin: const EdgeInsets.all(8),
child: SingleChildScrollView(
child: ListView(
shrinkWrap: true,
children: [
for (var i = 0; i < firebaseData!.length; i++) ...[
Card(
elevation: 4.0,
child: Column(
children: [
ListTile(
title: Text(
firebaseData[i]['Course Title'],
),
subtitle: Text(
firebaseData[i]['Course Group'],
),
trailing: IconButton(
onPressed: () {
Navigator.push(
context,
MaterialPageRoute(
builder: (context) =>
const AddCourse1(),
),
);
},
icon: const Icon(
Icons.add,
color: BBColors.bg1,
),
),
),
Container(
padding: EdgeInsets.all(16.0),
alignment: Alignment.centerLeft,
child: Text(
firebaseData[i]['Course Description'],
),
),
ButtonBar(
children: [
// IconButton(
//                                   onPressed: () {
//                                     Navigator.push(
//                                       context,
//                                       MaterialPageRoute(
//                                         builder: (context) => EditPage(
//                                           docID: firebaseData[i]['id'],
//                                         ),
//                                       ),
//                                     );
//                                   },
//                                   icon: const Icon(
//                                     Icons.edit,
//                                     color: Colors.orange,
//                                   ),
//                                 ),
IconButton(
onPressed: () {
_delete(firebaseData[i]['id']);
//print(firebaseData);
},
icon: const Icon(
Icons.delete,
color: Colors.red,
),
),
],
)
],
)),
], //this is loop
],
),
),
),
);
});
}
}

Answer 1

检查是否从firebaseData中的firebase获取数据变量，并像这样重构代码

subtitle: Text(firebaseData[i]['Course Group']??"Some Text",),

如果变量返回null然后输出硬编码的文本在右边，并保存应用程序免于崩溃。

Answer 2

任何与服务相关的信息都可能被接收为"null"。我建议使任何变量取决于互联网交互，可为空。所以当你使用它们时，你可以有占位符值。

例如

String? theUserNameFetchedFromInternet
// while using 
Text(theUserNameFetchedFromInternet ?? "john")

这种类型的处理可以防止大量的崩溃&这就是为什么dart是零安全的。