如何正确设置基于身份验证的导航

我一直在试图找出正确的方法,通过我的应用程序设置导航的基础上的价值 FirebaseAuth (即:仅在用户登录后导航)

User 类是我应用程序中的第一个页面,它在检查是否有 currentUser

    class User extends StatefulWidget {
      @override
      _UserState createState() => new _UserState();
    }

    class _UserState extends State<User> {
       FirebaseUser _user;


      @override
      initState() {
        super.initState();
        _checkUser();
      }

      @override
      Widget build(BuildContext context) {
        return _user == null? new SignInPage(): new HomePage();
      }

      _checkUser ()async {
        FirebaseUser user = await FirebaseAuth.instance.currentUser();
        setState((){
          this._user = user;
        });

      }
    }

当应用程序第一次启动时,它会加载 SignInPage

class SignInPage extends StatefulWidget {
  @override
  _SignInPageState createState() => new _SignInPageState();
}

class _SignInPageState extends State<SignInPage> {

  final FirebaseAuth _auth = FirebaseAuth.instance;
  final GoogleSignIn _googleSignIn = new GoogleSignIn();


   _handleSignIn() async{

    final GoogleSignInAccount googleUser = await _googleSignIn.signIn();
    final GoogleSignInAuthentication googleAuth =
    await googleUser.authentication;
    final FirebaseUser user = await _auth.signInWithGoogle(
      accessToken: googleAuth.accessToken,
      idToken: googleAuth.idToken,
    );
    assert(user.email != null);
    assert(user.displayName != null);
    assert(!user.isAnonymous);
    assert(await user.getToken() != null);
    print (user);

         }
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: new AppBar(
            title: new Text("Sign In"),
            centerTitle: true,
            leading: new Container(),
            backgroundColor: new Color.fromARGB(255,17,165,137)),
        body: new Container(
            child: new Center(
                child:
                new Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    new Row(
                      mainAxisAlignment: MainAxisAlignment.center,
                      mainAxisSize: MainAxisSize.max,
                      children: <Widget>[new RaisedButton(
                          child: new Container(
                              child: new Row(children: <Widget>[
                                new Container(
                                    child: new Image(
                                      image: new NetworkImage(
                                          "https://maxcdn.icons8.com/Share/icon/Logos//google_logo1600.png"),
                                    ),
                                    margin: new EdgeInsets.only(right: 8.0)),
                                new Text("Login with Google", style: new TextStyle(color: Colors.redAccent)),
                              ], mainAxisAlignment: MainAxisAlignment.center)),
                          onPressed: (){_handleSignIn().whenComplete((){Navigator.of(context).pushNamed("/Home");});}

                          ,color: Colors.white),
                      ],),
                    new Row(
                      mainAxisAlignment: MainAxisAlignment.center,
                      mainAxisSize: MainAxisSize.max,
                      )


                  ],
                )
            )
        )
    );
  }


}

HomePage 类,该类仅在验证身份验证后才可访问;我家有一个 RaisedButton 在中间,有一个 Text displayName 当前用户的,以及 onPressed() 调用 _signOut() 方法并返回到 SignInage

final GoogleSignIn _googleSignIn = new GoogleSignIn();


class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => new _HomePageState();
}

class _HomePageState extends State<HomePage> {
  FirebaseUser _currentUser;

  initState() {
    super.initState();
    _myUser();
  }
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Hello:)"),
      ),
      body: new Center(
        child: new RaisedButton(child: new Text(_currentUser.displayName),
            onPressed: (){_signOut().whenComplete((){Navigator.of(context).pushNamed("/SignInPage");});})
      ),
    );
  }
  _signOut() async {
    await FirebaseAuth.instance.signOut();

    await _googleSignIn.signOut();

    return "Signed out";
  }

   _myUser()async {
    FirebaseUser user = await FirebaseAuth.instance.currentUser();

    setState((){
      this._currentUser = user;
    });
  }


}

问题是:

1. 签名页 然而,正常情况下,当我成功登录到我的谷歌帐户时,它会冻结而不导航我,因为 displayName was called on Null

2. 重新启动应用程序并保留以前的身份验证会话时,应用程序会自动以 Home

3. 在同一个会话中,我单击 升起按钮 签名页 ,我尝试再次登录,我们回到了主要问题,它尝试将我导航到 家 获得。

这就是我对问题的理解,那么我在这里做错了什么,以及如何设计正确的流程?

附笔: 我想这样做的原因是因为我想使用 在我的 主页

更新:

控制台打印以下内容:

'package:test/SignInScreen.dart': Failed assertion: line 26: 'user.email != null': is not true.

assert 中的声明 _handleSignIn .

我也尝试过创建一个预主页,同样的问题也发生了,我不知道现在出了什么问题。