Null check operator used on a null value

Question

I am new to Flutter

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

Error

Null check operator used on a null value

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(),   )); }  

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(        ),     );   } }  

Error trace of the code

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/[email protected] 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}  

My flutter doctor ouput

 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. 

Can anyone provide a solution for this?

Answer 1

Any one who are using get_it package and having similar issue, here is the most simple solution. just add WidgetsFlutterBinding.ensureInitialized(); at the top of main function.

Change your main function like this :

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

Answer 2

Don't downgrade Flutter

Problem:

This error occurs when you use a bang operator (!) on a nullable instance which wasn't initialized.

For example:

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

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:

• Use a local variable

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

• Use ?. and ??

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

---

To answer your question:

You're using

Colors.blueAccent.shade50 

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. 

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 

---

For those who are using FutureBuilder/StreamBuilder:

You can solve the error in two ways:

• Specify a type to your 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();   }, ) 

• 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();   }, )