Null check operator used on a null value
FlutterDartDart Null-SafetyFlutter Problem Overview
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?
Flutter Solutions
Solution 1 - Flutter
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 50
th 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 100
th shade.
Colors.blueAccent[100]
// or
Colors.blue.shade100
FutureBuilder
/StreamBuilder
:
For those who are using 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 downcastObject
to your type, say aList
orMap
.FutureBuilder( future: _listOfInt(), builder: (_, snapshot) { if (snapshot.hasData) { var myList = snapshot.data! as List<int>; // <-- Your data using 'as' } return Container(); }, )
Solution 2 - Flutter
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());}
Solution 3 - Flutter
CopsOnRoad answer and only downgrade if that didn't work.
PreferSteps 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
Solution 4 - Flutter
For newbies like me to flutter, try restarting the app rather than hot reloading it.
Solution 5 - Flutter
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
Solution 6 - Flutter
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.
Solution 7 - Flutter
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.