null 값에 사용되는 null 검사 연산자

질문 제목:

null 값에 사용되는 null 검사 연산자

질문 내용:

나는 처음이다Flutter

간단한 flutter 앱을 실행할 때 이 오류가 발생했습니다. 이 오류가 발생한 이유를 알 수 없었습니다.

오류

Null check operator used on a null value

main.dart의 내 코드

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

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

login.dart의 내 코드

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(

      ),
    );
  }
}

코드의 오류 추적

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}

내 플러터 의사 출력

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.

누구든지 이에 대한 솔루션을 제공할 수 있습니까?

해결 답변:

Flutter를 다운그레이드하지 마세요.

문제:

!이 오류 는 초기화되지 않은 nullable 인스턴스에 bang 연산자( )를 사용할 때 발생합니다 .

예를 들어:

String? string; // Nullable String

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

솔루션:

로그를 열면 오류가 발생한 프로젝트의 파일을 가리키는 줄이 있어야 합니다.

null 값에 사용되는 null 검사 연산자

#0 메인 (package:example/main.dart:22:16)

거기에 있으면 다음 방법 중 하나를 사용하여 문제를 해결할 수 있습니다.

• 지역 변수 사용

  var s = string;
  if (s != null) {
    var len = s.length; // Safe 
  }
  

• 사용 ?.및 ??

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

---

스택 추적은 프로젝트에 속하지 않는 파일을 가리킬 수도 있습니다. 예를 들어:

1. 이용 Navigator하시거나MediaQuery

BuildContext이 오류는 에 비동기적 으로 액세스하려고 할 때도 발생합니다 .

mounted따라서 에 접근하기 전에 위젯이 있는지 먼저 확인해야 합니다 BuildContext.

Future<void> foo() async {
  // Some async operation
  await compute();  

  // Check `mounted` before accessing 'context'.
  if (mounted) {
    MediaQuery.of(context).size;
    Navigator.of(context).pop();  
  }
}

---

2. 이용하시는 분Color

사용 중

Colors.blueAccent.shade50

50그늘 이 없습니다 . 소스 코드를 살펴보면 다음을 찾을 수 있습니다.

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

null이 오류를 해결하려면 다음 음영 이 아닌 다른 색상을 사용해야 합니다 100.

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

---

FutureBuilder3. / 를 사용하시는 분들께 StreamBuilder:

두 가지 방법으로 오류를 해결할 수 있습니다.

• FutureBuilder/ 에 유형을 지정하십시오.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();
    },
  )
  

• as유형 으로 다운캐스팅 하는 데 사용 Object합니다(예: a List또는 ) Map.

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