flutter localization error when set title

localizaed.dart

import 'package:flutter/cupertino.dart';

class Localized {
  Localized(this.locale);

  final Locale locale;

  static Localized of(BuildContext context) => Localizations.of<Localized>(context, Localized);
  static Map<String, Map<String,String>> _v = {
    'en': {
      'title': 'hello world',
    },
    'ja': {
      'title': 'こんちは'
    }
  };
  String get title => _v[locale.languageCode]['title'];
}

class LocalizedDelagate extends LocalizationsDelegate<Localized> {
  const LocalizedDelagate();

  @override
  bool isSupported(Locale locale) => ['en','ja'].contains(locale.languageCode);
  @override
  Future<Localized> load(Locale locale) async => Localized(locale);
  @override
  bool shouldReload(LocalizationsDelegate old) => false;
}

main.dart

import 'package:calculator/src/localization/localized.dart';
import 'package:flutter/material.dart';
import 'package:calculator/src/pages/HomePage.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

void main() => runApp(StockCalcApp());

class StockCalcApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      localizationsDelegates: [
        const LocalizedDelagate(),
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate
      ],
      supportedLocales: [
        const Locale('en'),
        const Locale('ja'),
      ],
      onGenerateTitle: (BuildContext context) => Localized.of(context).title, // notthing problem
      theme: ThemeData(primarySwatch: Colors.pink),
      home: Scaffold(
        resizeToAvoidBottomPadding: false,
        appBar: AppBar(
          title: Text(Localized.of(context).title), // and same code but, error. when I comment this line then, nothing well.
          backgroundColor: Colors.pink[900],
          elevation: 0.0,
        ),
        body: StockHome(),
      ),
    );
  }
}

I cannot understand why to occur an error this message on the screen.

NoSuchMethodError: The getter 'title' was called on null.

I just do this example find on the web. and I think.. is very simple .. I think.
but seriously I cannot understand why to bring up this message on the android emulator.

When you call onGenerateTitle: (BuildContext context) => Localized.of(context).title, it uses a new BuildContext, which already contains the LocalizedDelagate(), so it can be called with Localized.of(context).

When you use it within the same build method, you refer to an instance of context before the LocalizedDelagate() was created, so Localized.of(context) doesn't return anything.

You can avoid this problem by creating a new Widget, which will have an updated BuildContext in it's build method, that will have access to Localized.
For example, create a new widget named HomeScreen

class StockCalcApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      localizationsDelegates: [
        const LocalizedDelagate(),
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate
      ],
      supportedLocales: [
        const Locale('en'),
        const Locale('ja'),
      ],
      onGenerateTitle: (BuildContext context) => Localized.of(context).title, 
      theme: ThemeData(primarySwatch: Colors.pink),
      home: HomeScreen(),
    );
  }
}

class HomeScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        resizeToAvoidBottomPadding: false,
        appBar: AppBar(
          title: Text(Localized.of(context).title), // this context will have access to Localized
          backgroundColor: Colors.pink[900],
          elevation: 0.0,
        ),
        body: StockHome(),
    );
  }
}