Локализация flutter приложения

   import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:shared_preferences/shared_preferences.dart';

class AppLocalizations {
  final Locale locale;

  AppLocalizations(this.locale);

  // Helper method to keep the code in the widgets concise
  // Localizations are accessed using an InheritedWidget "of" syntax
  static AppLocalizations of(BuildContext context) {
    return Localizations.of<AppLocalizations>(context, AppLocalizations);
  }

  // Static member to have a simple access to the delegate from the MaterialApp
  static const LocalizationsDelegate<AppLocalizations> delegate =
  _AppLocalizationsDelegate();

  Map<String, String> _localizedStrings;

  Future<bool> load() async {
    // Load the language JSON file from the "lang" folder
    final String jsonString =
        await rootBundle.loadString('i18n/${locale.languageCode}.json');

   final jsonMap = json.decode(jsonString) as Map<String, dynamic>;

    _localizedStrings = jsonMap.map((String key, value) {
      return MapEntry(key, value.toString());
    });

    return true;
  }


  // This method will be called from every widget which needs a localized text
  String translate(String key) {
    return _localizedStrings[key];
  }
}

class _AppLocalizationsDelegate
    extends LocalizationsDelegate<AppLocalizations> {
  // This delegate instance will never change (it doesn't even have fields!)
  // It can provide a constant constructor.
  const _AppLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) {
    // Include all of your supported language codes here
    // ignore: always_specify_types
    return ['en', 'pl'].contains(locale.languageCode);
  }

  @override
  Future<AppLocalizations> load(Locale locale) async {
    final AppLocalizations localizations = AppLocalizations(locale);
    await localizations.load();
    return localizations;
  }

  @override
  bool shouldReload(_AppLocalizationsDelegate old) => false;
}

class AppLanguage extends ChangeNotifier {
  Locale _appLocale = const Locale('en');

  Locale get appLocal => _appLocale ?? const Locale('en');

  // ignore: prefer_void_to_null
  Future<dynamic> fetchLocale() async {
    final SharedPreferences prefs = await SharedPreferences.getInstance();
    if (prefs.getString('language_code') == null) {
      _appLocale = const Locale('en');
      return null;
    }
    _appLocale = Locale(prefs.getString('language_code'));
    return null;
  }


  // ignore: avoid_void_async
  void changeLanguage(Locale type) async {
    final SharedPreferences prefs = await SharedPreferences.getInstance();
    if (_appLocale == type) {
      return;
    }
    if (type == const Locale('pl')) {
      _appLocale = const Locale('pl');
      await prefs.setString('language_code', 'pl');
      await prefs.setString('countryCode', '');
    }
    else {
      _appLocale = const Locale('en');
      await prefs.setString('language_code', 'en');
      await prefs.setString('countryCode', 'US');
    }
    notifyListeners();
  }
}

Вот весь код из файла, ошибка где-то в классе AppLanguage