Flutter-将getter调用为null

Question

我想将本地化添加到我的应用程序中（Flutter 1.10.7，频道beta），但是我遇到了麻烦。我必须将小部件包装在另一个类中才能调用它，否则会收到以下错误：

The following NoSuchMethodError was thrown building MyApp(dirty):
The getter 'hello' was called on null.
Receiver: null
Tried calling: hello

以下代码不起作用：

 class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        final i18n = I18n.delegate;
        return MaterialApp(
            theme: ThemeData(
              primarySwatch: Colors.blue,
            ),
            localizationsDelegates: [i18n],
            supportedLocales: i18n.supportedLocales,
            home: Text(I18n.of(context).hello));
      }
    }

此作品有效：

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final i18n = I18n.delegate;
    return MaterialApp(
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        localizationsDelegates: [i18n],
        supportedLocales: i18n.supportedLocales,
        home: MyWidget());
  }
}

class MyWidget extends StatelessWidget {   
  @override
  Widget build(BuildContext context) {
    return Text(
      I18n.of(context).hello,
    );
  }
}

有人可以解释一下为什么会这样吗？