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/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}

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

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? string; // Nullable String

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

Solutions:

Open the logs and there must be a line pointing to a file in your project where the error occurred:

Null check operator used on a null value

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

Once you are there, you can use any of the following ways to fix it:

• Use a local variable

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

• Use ?. and ??

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

---

The stack trace can also point to a file that doesn't belong to your project. For example:

1. For those who are using Navigator or MediaQuery

This error also occurs when you try to access a BuildContext asynchronously.

So, you should first check if the widget is mounted before accessing 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. For those who are using Color

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

---

3. 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();
    },
  )
  

Answer 2

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 3

Prefer CopsOnRoad answer and only downgrade if that didn't work.

Steps that we need to solve the above problem as follow

 - 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

When I tried flutter channel stable . I got another error since I had two flutters one is from snapd and another one is from git clone method.

then, I configured the SDK with the git clone. Finally, I used Git cloned SDK flutter to do all commands as follow

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

For newbies like me to flutter, try restarting the app rather than hot reloading it.

Answer 5

Provider Anti-Patterns

If we get this error outside of our package, then we may need to look into how we are using third-party dependencies. In my case, Provider anti-patterns were causing issues.

The lifecycle of a Provider object is different from a regular widget because it can be unexpectedly re-created or not created at all depending on usage.

Example 1

I encountered a simple reproducible example when initialising a ChangeNotifierProvider outside of the create: block.

Good

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

Bad

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

This is documented in the repository as an anti-pattern:

DON'T reuse an existing ChangeNotifier using the default constructor.

Example 2

Another anti-pattern that can cause similar side-effects is:

Do not use the .value constructor if you want to create an object, or you may otherwise have undesired side effects.

Good

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

Bad

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

Cause of error

sqflite databases should only have one call to openDatabase for consistency. Using ChangeNotifierProvider anti-patterns, I got this error because I was triggering openDatabase more than once. I thus triggered unhandled exceptions because sqflite internally returns null for openDatabase when we specify the same db path in repeated calls.

TIP: Check third-party documentation when you get this error outside of your package.

Answer 6

The issue might be related to the null safety that is getting integrated in dartlang. For now you can switch channels and use the stable channel:

flutter channel stable
flutter upgrade

Answer 7

This issue also occurs when you use a context that has already finished. For example, Navigator.pop(context), so you may check your code sequence again.

Answer 8

On my side, I fixed it by using late instead of ?

Previously: UserRepository? userRepository;

Replace it with: late UserRepository userRepository;

Answer 9

There might be different reasons for this. But for me, the reason was GetX. If anyone using GetX, make sure that you add init parameter inside your GetX. Here's an example:

late final DemoController _controller= Get.put(()=>DemoController());

GetX(
    init: _controller, 
    builder(_) =>...,
 );

Answer 10

If you are trying to set text with a nullable value then you can add this line to resolve this issue.

If the value of the name is null and you have to assign the nullable value then add

name ?? ""

Answer 11

You are getting this error because you haven't initialise your GetxController. Please initialise your controller where you are initialising all other controllers on app start.

Initialise like this

• Get.put(YourController()) immediately
• Get.lazyPut(()=> YourController()) when you need it

Answer 12

Was having the same issue and it happened the error was a result of Cloud Firestore rules. I had set the rule on request.time and the timestamp.date had expired. So you can check on that and set the rule either allow read, write: if true; or allow read, write: if request.auth != null;.

Answer 13

If run console not show detail error line in code, you can use dubug to know exactly which line of code it failed.