Flutter重定向到initState上的页面

我有一个需要登录才能继续的应用程序（例如Google）。

当需要身份验证时，我想重定向用户。

但是，当我运行一个Navigator.of(context).pushNamed("myroute")。我收到以下错误：

 ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 5624): The following assertion was thrown building _ModalScopeStatus(activE):
I/flutter ( 5624): setState() or markNeedsBuild() called during build.
I/flutter ( 5624): This Overlay widget cAnnot be marked as needing to build because the framework is already in the
I/flutter ( 5624): process of building widgets. A widget can be marked as needing to be built during the build phase
I/flutter ( 5624): only if one of its ancestors is currently building. This exception is allowed because the framework
I/flutter ( 5624): builds parent widgets before children,which means a dirty descendant will always be built.
I/flutter ( 5624): Otherwise,the framework might not visit this widget during this build phase.

这是示例代码

void main() {
    runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',theme: new ThemeData(
        priMarySwatch: Colors.blue,),home: new MyHomePage(title: 'Flutter Demo Home Page'),routes: <String,WidgetBuilder> {
        "login" : (BuildContext context) => new LoginPage(),}
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key,this.titlE}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  int _counter = 0;

    @override
    void initState() {
      super.initState();

      if(!isLoggedIn) {
        print("not logged in,going to login page");
        Navigator.of(context).pushNamed("login");
      }

    }


  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  void test() {
    print("Hello");
  }

  @override
  Widget build(BuildContext context) {

    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.titlE),body: new Center(
        child: new Text(
          'Button tapped $_counter time${ _counter == 1 ? '' : 's' }.',floaTingActionButton: new FloaTingActionButton(
        onPressed: _incrementCounter,tooltip: 'Increment',child: new Icon(Icons.add),// This Trailing comma makes auto-formatTing nicer for build methods.
    );
  }

}

class LoginPage extends StatefulWidget {
  LoginPage({Key key,this.titlE}) : super(key: key);

  final String title;

  @override
  _LoginPageState createState() => new _LoginPageState();
}

class _LoginPageState extends State<LoginPage> {
  @override
  Widget build(BuildContext context) {
    print("building login page");
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Sign up / Log In"),);
  }
}

我想我做错了，也许是中止小部件的构建导致了这一问题。但是，我怎么能做到这一点。

基本上：“我进入我的页面，如果未登录，请进入登录页面”

谢谢，Alexi