Flutter - Nullability problem with Locale

Below code has no compiler warnings or errors. Running it results in:

lib/main.dart:26:51: Error: The argument type 'Locale?' can't be assigned to the parameter type 'Locale' because 'Locale?' is nullable and 'Locale' isn't.
 - 'Locale' is from 'dart:ui'.
    thisAppsLocaleNotifier = ValueNotifier(window.locale);
                                                  ^
lib/main.dart:27:43: Error: Property 'languageCode' cannot be accessed on 'Locale?' because it is potentially null.
 - 'Locale' is from 'dart:ui'.
Try accessing using ?. instead.
    Localization.langCode = window.locale.languageCode;
                                          ^^^^^^^^^^^^
Failed to compile application.

Neither window nor locale could be null. No parameters here can be null. The error message is quite annoying because trying

thisAppsLocaleNotifier = ValueNotifier(window.locale ?? Locale('en'));

Results in a compiler warning:

The left operand can't be null, so the right operand is never executed.  Try removing the operator and the right operand.

This is the code:

import 'dart:ui';

import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

import 'main.i18n.dart';

/// Supported locales. First entry `en` is default.
const List<Locale> supportedLocales = [
  Locale('en', 'US'),
  Locale('de', 'DE'),
  Locale('es', 'ES'),
];

late ValueNotifier<Locale> thisAppsLocaleNotifier;

/// Entry point for example application
void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  MyApp() {
    thisAppsLocaleNotifier = ValueNotifier(window.locale);
    Localization.langCode = window.locale.languageCode;
  }

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return ValueListenableBuilder(
      valueListenable: thisAppsLocaleNotifier,
      builder: (BuildContext context, Locale thisAppsLocale, Widget? child) =>
          MaterialApp(
        home: MyHomePage(),
        locale: thisAppsLocale,
        localizationsDelegates: [
          GlobalMaterialLocalizations.delegate,
          GlobalWidgetsLocalizations.delegate,
          GlobalCupertinoLocalizations.delegate,
        ],
        supportedLocales: supportedLocales,
        theme: ThemeData(
          primarySwatch: Colors.orange,
        ),
        title: 'input_country',
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    print('_MyHomePageState build');
    return Scaffold(
      appBar: AppBar(
        title: Text('App Title'.i18n),
      ),
      body: Center(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          mainAxisAlignment: MainAxisAlignment.start,
          children: <Widget>[
            Text('thisAppsLocale = ${thisAppsLocaleNotifier.value} / langCode ='
                ' ${Localization.langCode}'),
            ButtonBar(
              alignment: MainAxisAlignment.center,
              children: [
                ElevatedButton(
                  onPressed: () => setLocale(Locale('de')),
                  child: Text('de'),
                ),
                ElevatedButton(
                  onPressed: () => setLocale(Locale('en')),
                  child: Text('en'),
                ),
                ElevatedButton(
                  onPressed: () => setLocale(Locale('es')),
                  child: Text('es'),
                ),
              ],
            )
          ],
        ),
      ),
    );
  }

  void setLocale(Locale newLocale) {
    Localization.langCode = newLocale.languageCode;
    thisAppsLocaleNotifier.value = newLocale;
  }
}

And this is pubspec.yaml

name: qq
description: A new Flutter application.
publish_to: 'none'
version: 1.0.0+1

environment:
  sdk: ">=2.12.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter

dev_dependencies:
  flutter_test:
    sdk: flutter

flutter:
  uses-material-design: true

What is required to run the code in null-safety mode?

This is the localization class main.i18n.dart:

extension Localization on String {
  static String langCode = 'en';

  String get i18n => localize(this, _t);

  static final Map<String, Map<String, String>> _t = {
    'All': {
      'de': 'Alle',
      'es': 'Todas',
    },
    'Example': {
      'de': 'Beispiel',
      'es': 'Ejemplo',
    },
    'Languages': {
      'de': 'Sprachen',
      'es': 'Idiomas',
    },
    'Selectables': {
      'de': 'Einträge',
      'es': 'Entradas',
    },
  };

  String localize(String english, Map<String, Map<String, String>> t10ns) {
    if (langCode != 'en') {
      Map<String, String>? _t10ns = t10ns[english];
      if (_t10ns == null) {
        print('No translations found for "$english"');
      } else {
        String? translated = _t10ns[langCode];
        if (translated == null) {
          print('Translation to language "$langCode" missing for "$english"');
        } else {
          return translated;
        }
      }
    }
    return english;
  }
}

Output of Flutter doctor:

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, 2.0.3, on Microsoft Windows [Version 10.0.19042.867], locale de-DE)
[√] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
[√] Chrome - develop for the web
[√] Android Studio (version 4.1.0)
[√] Connected device (3 available)

• No issues found!