Unhandled Exception: NoSuchMethodError: The method 'findAncestorStateOfType' was called on null

问题

Because of the following error, my app is stuck at HandleMainScreen. It Navigated properly for the first time but gives error in the console but when I hot restart it Navigation doesn't work and app stuck at HandleMainScreen.

E/flutter ( 8785): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'findAncestorStateOfType' was called on null.
E/flutter ( 8785): Receiver: null
E/flutter ( 8785): Tried calling: findAncestorStateOfType<NavigatorState>()
E/flutter ( 8785): #0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:53:5)
E/flutter ( 8785): #1      Navigator.of 
package:flutter/…/widgets/navigator.dart:2115
E/flutter ( 8785): #2      _HandleMainScreenState.checkAuthentication.<anonymous closure> 
package:monktree/main.dart:58
E/flutter ( 8785): #3      _rootRunUnary  (dart:async/zone.dart:1192:38)
E/flutter ( 8785): #4      _CustomZone.runUnary  (dart:async/zone.dart:1085:19)
E/flutter ( 8785): #5      _CustomZone.runUnaryGuarded  (dart:async/zone.dart:987:7)
E/flutter ( 8785): #6      _BufferingStreamSubscription._sendData  (dart:async/stream_impl.dart:339:11)
E/flutter ( 8785): #7      _BufferingStreamSubscription._add  (dart:async/stream_impl.dart:266:7)

Following code is what I am using. If you have any better way to handle user long kindly answer it on this question.

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      builder: (context, child) {
        return ScrollConfiguration(
          behavior: MyBehavior(),
          child: child,
        );
      },
      home: HandleMainScreen(),
      debugShowCheckedModeBanner: false,
    );
  }
}
class MyBehavior extends ScrollBehavior {
  @override
  Widget buildViewportChrome(
      BuildContext context, Widget child, AxisDirection axisDirection) {
    return child;
  }
}
class HandleMainScreen extends StatefulWidget {
  @override
  _HandleMainScreenState createState() => _HandleMainScreenState();
}

class _HandleMainScreenState extends State<HandleMainScreen> {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  bool isLoggedIn;
  FirebaseUser _user;
  String error;

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

  checkAuthentication() async {
    _auth.onAuthStateChanged.listen((user) {
      _user = user;
      if (user == null) {
        Navigator.of(context).pushReplacement(
            MaterialPageRoute(builder: (context) => LoginPage()));
      } else if (user != null) {
        Navigator.of(context).pushReplacement(
            MaterialPageRoute(builder: (context) => HomePage(user)));
      }
    }).onError((e) {
      error = e.toString();
      print("Error in HandleMainScreen\n" + e.toString());
    });
  }

  @override
  Widget build(BuildContext context) {
    Constants.allocateSize(size: MediaQuery.of(context).size);
    return Scaffold(
      body: Center(
        child: CircularProgressIndicator(),
      ),
    );
  }
}

回答1:

Change this:

  checkAuthentication() async {
    _auth.onAuthStateChanged.listen((user) {
      _user = user;
      if (user == null) {
        Navigator.of(context).pushReplacement(
            MaterialPageRoute(builder: (context) => LoginPage()));
      } else if (user != null) {
        Navigator.of(context).pushReplacement(
            MaterialPageRoute(builder: (context) => HomePage(user)));
      }
    }).onError((e) {
      error = e.toString();
      print("Error in HandleMainScreen\n" + e.toString());
    });
  }

into this:

  checkAuthentication() async {
    _auth.onAuthStateChanged.listen((user) {
      _user = user;
      if (user == null) {
        Navigator.pushReplacement(
          context,
          MaterialPageRoute(builder: (context) => LoginPage()),
        );
      } else if (user != null) {
         Navigator.pushReplacement(
          context,
          MaterialPageRoute(builder: (context) => HomePage(user)),
        );
      }
    }).onError((e) {
      error = e.toString();
      print("Error in HandleMainScreen\n" + e.toString());
    });
  }

来源：https://stackoverflow.com/questions/62281488/unhandled-exception-nosuchmethoderror-the-method-findancestorstateoftype-was