用于空值的空值检查运算符

Question

我是Flutter的新手

当我运行我的简单颤振应用程序时出现此错误。我不知道为什么会发生这个错误。

错误

Null check operator used on a null value

我在 main.dart 中的代码

import 'package:flutter/material.dart';
import './ui/login.dart';

void main() {
  
  runApp(new MaterialApp(
    title: "Login Template",
    home: new Login(),
  ));
}

我在 login.dart 中的代码

import 'package:flutter/material.dart';

class Login extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return new LoginState();
  }
}

class LoginState extends State<Login> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Login"),
        centerTitle: true,
        backgroundColor: Colors.blueAccent.shade50,
      ),
      backgroundColor: Colors.blueGrey,
      body: new Container(

      ),
    );
  }
}

代码的错误跟踪

Running Gradle task 'assembleDebug'...
✓ Built build/app/outputs/flutter-apk/app-debug.apk.
Installing build/app/outputs/flutter-apk/app.apk...
Waiting for SM J710F to report its views...
D/vndksupport(29495): Loading /vendor/lib/hw/android.hardware.graphics.mapper@2.0-impl.so from current namespace instead of sphal namespace.
Debug service listening on ws://127.0.0.1:39899/9RorUiKtUb4=/ws
Syncing files to device SM J710F...
D/ViewRootImpl@4ac1ef4[MainActivity](29495): MSG_RESIZED_REPORT: frame=Rect(0, 0 - 720, 1280) ci=Rect(0, 48 - 0, 582) vi=Rect(0, 48 - 0, 582) or=1
D/ViewRootImpl@4ac1ef4[MainActivity](29495): MSG_WINDOW_FOCUS_CHANGED 1
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@3049fea nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
I/InputMethodManager(29495): startInputInner - mService.startInputOrWindowGainedFocus
D/InputTransport(29495): Input channel constructed: fd=96
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@aad92db nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
D/ViewRootImpl@4ac1ef4[MainActivity](29495): MSG_RESIZED: frame=Rect(0, 0 - 720, 1280) ci=Rect(0, 48 - 0, 0) vi=Rect(0, 48 - 0, 0) or=1
D/ViewRootImpl@4ac1ef4[MainActivity](29495): Relayout returned: old=[0,0][720,1280] new=[0,0][720,1280] result=0x1 surface={valid=true 3791374336} changed=false
D/libGLESv2(29495): STS_GLApi : DTS, ODTC are not allowed for Package : com.sivaram.login_template

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
Null check operator used on a null value
Login file:///home/kadavul/IdeaProjects/login_template/lib/main.dart:8:15
════════════════════════════════════════════════════════════════════════════════════════════════════
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@a0ff0af nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
I/InputMethodManager(29495): startInputInner - mService.startInputOrWindowGainedFocus
D/InputTransport(29495): Input channel constructed: fd=87
D/InputTransport(29495): Input channel destroyed: fd=96
D/SurfaceView(29495): windowStopped(true) false 77b9092 of ViewRootImpl@4ac1ef4[MainActivity]
D/SurfaceView(29495): BG show() Surface(name=Background for - SurfaceView - com.sivaram.login_template/com.sivaram.login_template.MainActivity@77b9092@0) io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/SurfaceView(29495): surfaceDestroyed 1 #1 io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@a78fcbc nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
I/InputMethodManager(29495): startInputInner - mService.startInputOrWindowGainedFocus
D/InputTransport(29495): Input channel constructed: fd=91
D/InputTransport(29495): Input channel destroyed: fd=87
D/SurfaceView(29495): windowStopped(false) true 77b9092 of ViewRootImpl@4ac1ef4[MainActivity]
D/SurfaceView(29495): BG show() Surface(name=Background for - SurfaceView - com.sivaram.login_template/com.sivaram.login_template.MainActivity@77b9092@1) io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
V/Surface (29495): sf_framedrop debug : 0x4f4c, game : false, logging : 0
D/SurfaceView(29495): surfaceCreated 1 #1 io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/mali_winsys(29495): EGLint new_window_surface(egl_winsys_display *, void *, EGLSurface, EGLConfig, egl_winsys_surface **, egl_color_buffer_format *, EGLBoolean) returns 0x3000,  [720x1280]-format:1
D/SurfaceView(29495): surfaceChanged (720,1280) 1 #1 io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/SurfaceView(29495): BG destroy() Surface(name=Background for - SurfaceView - com.sivaram.login_template/com.sivaram.login_template.MainActivity@77b9092@0) io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/ViewRootImpl@4ac1ef4[MainActivity](29495): Relayout returned: old=[0,0][720,1280] new=[0,0][720,1280] result=0x3 surface={valid=true 3791374336} changed=false
D/ViewRootImpl@4ac1ef4[MainActivity](29495): MSG_RESIZED_REPORT: frame=Rect(0, 0 - 720, 1280) ci=Rect(0, 48 - 0, 0) vi=Rect(0, 48 - 0, 0) or=1
V/InputMethodManager(29495): Starting input: tba=android.view.inputmethod.EditorInfo@7ed1445 nm : com.sivaram.login_template ic=null
D/InputMethodManager(29495): startInputInner - Id : 0
I/InputMethodManager(29495): startInputInner - mService.startInputOrWindowGainedFocus
D/InputTransport(29495): Input channel constructed: fd=92
D/InputTransport(29495): Input channel destroyed: fd=91
D/SurfaceView(29495): windowStopped(true) false 77b9092 of ViewRootImpl@4ac1ef4[MainActivity]
D/SurfaceView(29495): BG show() Surface(name=Background for - SurfaceView - com.sivaram.login_template/com.sivaram.login_template.MainActivity@77b9092@1) io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}
D/SurfaceView(29495): surfaceDestroyed 1 #1 io.flutter.embedding.android.FlutterSurfaceView{77b9092 V.E...... ........ 0,0-720,1280}

我的颤振医生输出

fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Doctor summary (to see all details, run flutter doctor -v):
Failed to find the latest git commit date: VersionCheckError: Command exited with code 128: git -c log.showSignature=false log -n 1 --pretty=format:%ad --date=iso
Standard out: 
Standard error: fatal: your current branch 'master' does not have any commits yet

Returning 1970-01-01 05:30:00.000 instead.
[✓] Flutter (Channel unknown, 0.0.0-unknown, on Linux, locale en_US.UTF-8)
 
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
[✓] Android Studio (version 4.0)
[!] VS Code (version 1.50.0)
    ✗ Flutter extension not installed; install from
      https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter
[✓] Connected device (1 available)

! Doctor found issues in 1 category.

谁能为此提供解决方案？

Answer 1

不要降级 Flutter

问题：

当您在未初始化的可为空的实例上使用 bang 运算符 (!) 时会发生此错误。

例如：

String? foo; // Nullable String

void main() {
  var len = foo!.length; // Runtime error: Null check operator used on a null value
}

解决方案：

您需要找出在代码中使用 bang 运算符的位置。到达那里后，您可以使用以下任何一种解决方案：

• 使用局部变量

  var f = foo;
  if (f != null) {
    var len = f.length; // Safe 
  }
  

• 使用 ?. 和 ??

  var len = foo?.length ?? 0; // Provide a default value if foo was null.
  

---

回答您的问题：

你正在使用

Colors.blueAccent.shade50

没有50th 阴影。如果你查看源代码，你会发现：

Color get shade50 => this[50]!; // <-- This bang operator is causing the error.

要解决此错误，您应该使用不是null 的其他颜色，可能是100th 阴影。

Colors.blueAccent[100]
// or
Colors.blue.shade100

---

对于正在使用FutureBuilder/StreamBuilder的人：

您可以通过两种方式解决该错误：

• 为您的FutureBuilder/StreamBuilder指定一个类型

  FutureBuilder<List<int>>( // <-- type 'List<int>' is specified.
    future: _listOfInt(),
    builder: (_, snapshot) {
      if (snapshot.hasData) {
        List<int> myList = snapshot.data!; // <-- Your data
      }
      return Container();
    },
  )
  

• 使用as 将Object 向下转换为您的类型，例如List 或Map。

  FutureBuilder(
    future: _listOfInt(),
    builder: (_, snapshot) {
      if (snapshot.hasData) {
        var myList = snapshot.data! as List<int>; // <-- Your data using 'as'
      }
      return Container();
    },
  )
  

Answer 2

任何使用 get_it 包并遇到类似问题的人，这里是最简单的解决方案。 只需在主函数顶部添加 WidgetsFlutterBinding.ensureInitialized(); 即可。

像这样改变你的主要功能：

Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
await di.init()
runApp(MyApp());}

Answer 3

首选CopsOnRoad answer，只有在不起作用时才降级。

解决上述问题需要的步骤如下

 - flutter channel stable 
 - flutter upgrade
 - flutter pub cache repair //To perform a clean reinstall of the packages in your system cache, use pub cache repair
 - flutter clean //flutter clean will delete the /build folder

当我尝试flutter channel stable 时。我遇到了另一个错误，因为我有两个颤振，一个来自 snapd，另一个来自 git clone 方法。

然后，我使用 git clone 配置了 SDK。最后，我使用 Git 克隆的 SDK Flutter 执行所有命令如下

flutter channel stable 
 - ~/flutter/bin/flutter upgrade
 - ~/flutter/bin/flutter pub cache repair //To perform a clean reinstall of the packages in your system cache, use pub cache repair
 - ~/flutter/bin/flutter clean //flutter clean will delete the /build folder

Answer 4

该问题可能与dartlang 中集成的空安全性有关。现在您可以切换频道并使用稳定频道：

flutter channel stable
flutter upgrade

Answer 5

对于像我这样的新手，请尝试重新启动应用程序而不是热重新加载它。

Answer 6

Provider 反模式

如果我们在我们的包之外得到这个错误，那么我们可能需要看看我们是如何使用第三方依赖的。就我而言，Provider 反模式导致了问题。

Provider 对象的生命周期与常规小部件不同，因为它可以是 unexpectedly re-created or not created at all，具体取决于使用情况。

示例 1

在create: 块之外初始化ChangeNotifierProvider 时，我遇到了一个简单的可重现示例。

好

ChangeNotifierProvider(
  create: (_) => ToDoContainerModel(),
  builder: (context, _) {
    return home;
}));

不好

ChangeNotifierProvider(
  create: (_) => model,
  builder: (context, _) {
    return home;
}));

这在the repository 中被记录为反模式：

不要使用默认构造函数重用现有的 ChangeNotifier。

示例 2

另一个可能导致类似副作用的反模式是：

如果您想创建一个对象，请不要使用.value 构造函数，否则您可能会产生不希望的副作用。

好

ChangeNotifierProvider.value(
  create: (_) => MyModel(),
  child: ...
)

不好

ChangeNotifierProvider.value(
  value: MyModel(),
  child: ...
)

错误原因

sqflite 数据库应该只调用一次openDatabase 以保持一致性。使用ChangeNotifierProvider 反模式，我得到了这个错误，因为我不止一次触发了openDatabase。因此，我触发了未处理的异常，因为当我们在重复调用中指定相同的数据库路径时，sqflite 在内部为openDatabase 返回 null。

提示：当您在包之外遇到此错误时，请检查第三方文档。