Non-nullable instance field '_localizedStrings' must be initialized - flutter

Question

To be honest, I didn't meet this kind of error before. I am following a tutorial which you can see here:
https://resocoder.com/2019/06/01/flutter-localization-the-easy-way-internationalization-with-json/
Here is my code:
MAIN.dart:
import 'app_localizations.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
supportedLocales: [Locale('en', 'US'), Locale('vi', 'VN')],
localizationsDelegates: [
// THIS CLASS WILL BE ADDED LATER
// A class which loads the translations from JSON files
AppLocalizations.delegate,
// Built-in localization of basic text for Material widgets
GlobalMaterialLocalizations.delegate,
// Built-in localization for text direction LTR/RTL
GlobalWidgetsLocalizations.delegate,
],
localeResolutionCallback: (locale, supportedLocales) {
for (var supportedLocale in supportedLocales) {
if (supportedLocale.languageCode == locale!.languageCode &&
supportedLocale.countryCode == locale.countryCode) {
return supportedLocale;
}
}
return supportedLocales.first;
},
home: MyHomePage(),
);
}
}
APP_LOCALIZATIONS.dart:
import 'dart:async';
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
class AppLocalizations {
final Locale locale;
//error right here
AppLocalizations(this.locale);
//error right here
//modify
static AppLocalizations? of(BuildContext context) {
return Localizations.of<AppLocalizations>(context, AppLocalizations);
}
//modify
static const LocalizationsDelegate<AppLocalizations> delegate =
_AppLocalizationsDelegate();
Map<String, String> _localizedStrings;
Future<bool> load() async {
String jsonString =
await rootBundle.loadString('lang/${locale.languageCode}.json');
Map<String, dynamic> jsonMap = json.decode(jsonString);
_localizedStrings = jsonMap.map((key, value) {
return MapEntry(key, value.toString());
});
return true;
}
//modify
String? translate(String key) {
return _localizedStrings[key];
}
}
//modify
class _AppLocalizationsDelegate
extends LocalizationsDelegate<AppLocalizations> {
const _AppLocalizationsDelegate();
#override
bool isSupported(Locale locale) {
return ['en', 'vi'].contains(locale.languageCode);
}
#override
Future<AppLocalizations> load(Locale locale) async {
AppLocalizations localizations = new AppLocalizations(locale);
await localizations.load();
return localizations;
}
#override
bool shouldReload(_AppLocalizationsDelegate old) => false;
}
Some code here is modified because of some error so it is a little different from the tutorial (to fix error.). So maybe it is the reason. I put some comment about that modification up there.
P/S: I tried searching for other answer about this but I couldn't understand much so pls explains for me!

Answer 1

Just initialize _localizedStrings to an empty map.
Map<String, String> _localizedStrings = {};
This way it will never be null even if it doesn't have data yet.
The other option is to add the late modifier before declaration which tells the compiler it will be initialized later on.
late Map<String, String> _localizedStrings;