Existing l10n.yaml file is mandatory or null check assertion is thrown in AppLocalizations.of #120457
Description
Steps to Reproduce
Following the documentation to install internationalization and localization except not adding the l10n.yaml file results in a null check operator exception when localizations are attempted to be used to set aMaterialApp.title property. Otherwise sensible defaults allow translations to work in the rest of the app. None of the default properties (arb-dir, template-arb-file, or output-localization-file) of the example l10n.yaml seem to have anything to do with using a null check operator versus returning a nullable type.
-
Follow guide here to create a new project (
flutter create test) and add internationalization. -
Follow guide here to add localizations, but do not add a
l10n.yamlfile (Step 3). -
Add
app_en.arblocalization file (Step 4) as described. -
Run
flutter gen-l10n -
Modify
main.dartas described by the guide. -
Call
AppLocalizations.ofin MyHomePage, seemain.dartcode file below. -
Run app and observe application runs with translated text.
-
Call
AppLocalizations.ofinMaterialApp.title, seemain.dartcode file below. -
Run app and observe application hits an exception of "Null check operator used on a null value" in the
ofmethod of theAppLocalizationsclass:
static AppLocalizations of(BuildContext context) {
return Localizations.of<AppLocalizations>(context, AppLocalizations)!;
}
- Add the
l10n.yamlas described in step 3 of the localizations guide. - Run
flutter gen-l10n - Observe that null check operator has disappeared from the
app_localizations.dartfile.
static AppLocalizations? of(BuildContext context) {
return Localizations.of<AppLocalizations>(context, AppLocalizations);
}
- Run app and observe application runs without an exception.
Expected results:
Without a l10n.yaml file the AppLocalizations class file generated should still return a nullable type not throw a null check operator exception. Or a much stronger warning should be placed in the documented steps that broken behavior might occur without this file.
Actual results:
Application shows red screen with yellow text of "Null check operator used on a null value"
Debug window shows:
Exception has occurred.
_CastError (Null check operator used on a null value)
Code sample
import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';
import 'package:flutter_gen/gen_l10n/app_localizations.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
Widget build(BuildContext context) {
return MaterialApp(
title: AppLocalizations.of(context)?.helloWorld ?? '', // BREAKS IF l10n.yaml FILE NOT PROVIDED
localizationsDelegates: const [
AppLocalizations.delegate,
GlobalMaterialLocalizations.delegate,
GlobalWidgetsLocalizations.delegate,
GlobalCupertinoLocalizations.delegate,
],
supportedLocales: const [
Locale('en'), // English
Locale('es'), // Spanish
],
home: const MyHomePage(),
);
}
}
class MyHomePage extends StatelessWidget {
const MyHomePage({super.key});
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title:
Text(AppLocalizations.of(context)?.helloWorld.toUpperCase() ?? ''),
),
body: Center(
child: Text(
AppLocalizations.of(context)?.helloWorld.toLowerCase() ?? '')),
);
}
}
Logs
[ +21 ms]
══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following _CastError was thrown building MyApp(dirty):
Null check operator used on a null value
The relevant error-causing widget was:
MyApp MyApp:file:///C:/Users/*********/Documents/workspace/test-flutter/test/lib/main.dart:6:16
When the exception was thrown, this was the stack:
#0 AppLocalizations.of (package:flutter_gen/gen_l10n/app_localizations.dart:67:73)
#1 MyApp.build (package:test/main.dart:16:31)
#2 StatelessElement.build (package:flutter/src/widgets/framework.dart:5038:49)
#3 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4968:15)
#4 Element.rebuild (package:flutter/src/widgets/framework.dart:4690:5)
#5 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4950:5)
#6 ComponentElement.mount (package:flutter/src/widgets/framework.dart:4944:5)
#7 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3953:16)
#8 Element.updateChild (package:flutter/src/widgets/framework.dart:3682:18)
#9 RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1176:16)
#10 RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:1145:5)
#11 RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:1092:18)
#12 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2682:19)
#13 RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:1091:13)
#14 WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:926:7)
#15 WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:906:7)
#19 _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:192:26)
(elided 3 frames from class _Timer and dart:async-patch)
════════════════════════════════════════════════════════════════════════════════════════════════════
flutter analyze
Analyzing test...
warning - The receiver can't be null, so the null-aware operator '?.' is unnecessary - lib\main.dart:16:42 - invalid_null_aware_operator
warning - The receiver can't be null, so the null-aware operator '?.' is unnecessary - lib\main.dart:40:46 - invalid_null_aware_operator
warning - The receiver can't be null, so the null-aware operator '?.' is unnecessary - lib\main.dart:44:43 - invalid_null_aware_operator
3 issues found. (ran in 1.3s)
[√] Flutter (Channel stable, 3.7.1, on Microsoft Windows [Version
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• Flutter version 3.7.1 on channel stable at C:\src\flutter
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