用于空值的空检查运算符 [英] Null check operator used on a null value

问题描述

我是 Flutter

我在运行简单的 flutter APP 时遇到了这个错误.我无法弄清楚为什么会发生此错误.

I got this error when I run my simple flutter APP. I could not figure out why this error occurred.

错误

用于空值的空检查运算符

我在 main.dart 中的代码

My code in main.dart

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

My code in login.dart

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/[e-mail] 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?

Can anyone provide a solution for this?

推荐答案

不要降级 Flutter

问题:

在未初始化的可空实例上使用 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:

Solutions:

You need to find out where you're using the bang operator in your code. Once you are there, you can use any of the following solutions:

• 使用局部变量

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

使用 ?. 和 ??

Use ?. and ??

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

您正在使用

Colors.blueAccent.shade50

没有 50th 阴影.如果您查看源代码，您会发现:

which doesn't have 50th shade. If you look into the source code, you'd find:

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

要解决此错误，您应该使用其他一些不是 null 的颜色，也许是 100th 阴影.

To solve this error, you should use some other color which is not null, maybe the 100th shade.

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

---

对于那些使用 FutureBuilder/StreamBuilder 的人:

您可以通过两种方式解决错误:

---

For those who are using FutureBuilder/StreamBuilder:

You can solve the error in two ways:

• 为您的 FutureBuilder/StreamBuilder

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，比如 List 或 Map.

Use as to downcast Object to your type, say a List or Map.

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

这篇关于用于空值的空检查运算符的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！