Firebase Authenticationでログアウト時に再ログインできなくなる

FlutterでFirebaseを使ってID、PWによるログイン処理を実装しています。
そこで、ログインはうまくいくのですが、ログアウトしてログインページに戻ったあとに再ログインできないという事象に悩まされています。
具体的には認証情報を入力してLoginボタンを押してもhomepage.dartに画面遷移しません。

基本的には、こちらを参考に実装しています。

エラーです。

I/flutter (10611): Error: NoSuchMethodError: The method 'signIn' was called on null.
I/flutter (10611): Receiver: null
I/flutter (10611): Tried calling: signIn("IDが表示されている", "<PWが表示されている>")
E/flutter (10611): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: Class 'NoSuchMethodError' has no instance getter 'message'.
E/flutter (10611): Receiver: Instance of 'NoSuchMethodError'
E/flutter (10611): Tried calling: message
E/flutter (10611): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
E/flutter (10611): #1      _LoginSignupPageState.validateAndSubmit.<anonymous closure> (package:flutter_app/login_signup_page.dart:78:29)
E/flutter (10611): #2      State.setState (package:flutter/src/widgets/framework.dart:1148:30)
E/flutter (10611): #3      _LoginSignupPageState.validateAndSubmit (package:flutter_app/login_signup_page.dart:76:9)
E/flutter (10611): #4      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:706:14)
E/flutter (10611): #5      _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:789:36)
E/flutter (10611): #6      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
E/flutter (10611): #7      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:486:11)
E/flutter (10611): #8      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:264:5)
E/flutter (10611): #9      BaseTapGestureRecognizer.acceptGesture (package:flutter/src/gestures/tap.dart:236:7)
E/flutter (10611): #10     GestureArenaManager.sweep (package:flutter/src/gestures/arena.dart:156:27)
E/flutter (10611): #11     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:222:20)
E/flutter (10611): #12     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
E/flutter (10611): #13     GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
E/flutter (10611): #14     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
E/flutter (10611): #15     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
E/flutter (10611): #16     _rootRunUnary (dart:async/zone.dart:1138:13)
E/flutter (10611): #17     _CustomZone.runUnary (dart:async/zone.dart:1031:19)
E/flutter (10611): #18     _CustomZone.runUnaryGuarded (dart:async/zone.dart:933:7)
E/flutter (10611): #19     _invoke1 (dart:ui/hooks.dart:273:10)
E/flutter (10611): #20     _dispatchPointerDataPacket (dart:ui/hooks.dart:182:5)

ソースです。★をつけた箇所がデバッグした結果、怪しいと感じた箇所です。

main.dart

void main() {
  // 省略
}

class MyApp extends StatelessWidget {
  Widget build(BuildContext context) {
    return MaterialApp(
        // 省略,
        home: RootPage(auth: Auth()));
  }
}

root_page.dart

class RootPage extends StatefulWidget {
  RootPage({this.auth});

　//★authインスタンスは重複していないか
  final BaseAuth auth;
  State<StatefulWidget> createState() => _RootPageState();
}

class _RootPageState extends State<RootPage> {
  AuthStatus authStatus = AuthStatus.NOT_DETERMINED;
  String _userId = "";

  void initState() {
    super.initState();
    widget.auth.getCurrentUser().then((user) {
      setState(() {
        if (user != null) {
          _userId = user?.uid;
        }
        authStatus =
            user?.uid == null ? AuthStatus.NOT_LOGGED_IN : AuthStatus.LOGGED_IN;
      });
    });
  }

  void loginCallback() {
　　// 省略
  }

　//★本処理は適切に呼ばれているか
  void logoutCallback() {
    setState(() {
      authStatus = AuthStatus.NOT_LOGGED_IN;
      _userId = "";
    });
  }

  Widget buildWaitingScreen() {
    return Scaffold(
      // 省略
    );
  }

  Widget build(BuildContext context) {
    switch (authStatus) {
      case AuthStatus.NOT_DETERMINED:
        return buildWaitingScreen();
        break;
      case AuthStatus.NOT_LOGGED_IN:
        return LoginSignupPage(
          auth: widget.auth,
          loginCallback: loginCallback,
        );
        break;
      case AuthStatus.LOGGED_IN:
        if (_userId.length > 0 && _userId != null) {
          return HomePage(
            userId: _userId,
            auth: widget.auth,
            logoutCallback: logoutCallback,
          );
        } else
          return buildWaitingScreen();
        break;
      default:
        return buildWaitingScreen();
    }
  }
}

login_signup_page.dart

class LoginSignupPage extends StatefulWidget {
  LoginSignupPage({this.auth, this.loginCallback});

  final BaseAuth auth;
  final VoidCallback loginCallback;
  State<StatefulWidget> createState() => _LoginSignupPageState();
}

class _LoginSignupPageState extends State<LoginSignupPage> {
  final _formKey = GlobalKey<FormState>();
  String _groupId;
  String _displayName;
  String _email;
  String _password;
  String _errorMessage;
  bool _isLoginForm;
  bool _isLoading;

  bool validateAndSave() {
    final form = _formKey.currentState;
    if (form.validate()) {
      form.save();
      return true;
    }
    return false;
  }

  void validateAndSubmit() async {
    setState(() {
      _errorMessage = "";
      _isLoading = true;
    });
    if (validateAndSave()) {
      String userId = "";
      try {
        if (_isLoginForm) {
　　　　　//★このtry文でエラーが発生しているように見える
          userId = await widget.auth.signIn(_email, _password);
        } else {
          userId = await widget.auth.signUp(_email, _password);
          Firestore.instance.collection('users').document(userId).setData({
            // 省略
          });
          widget.auth.sendEmailVerification();
          _showVerifyEmailSentDialog();
        }
        setState(() {
          _isLoading = false;
        });

        if (userId.length > 0 && userId != null && _isLoginForm) {
          widget.loginCallback();
        }
      } catch (e) {
        //★ここでeがnullになっていることが原因？
        setState(() {
          _isLoading = false;
          _errorMessage = e.message;
          _formKey.currentState.reset();
        });
      }
    }
  }

  @override
  void initState() {
    _errorMessage = "";
    _isLoading = false;
    _isLoginForm = true;
    super.initState();
  }
  void resetForm() {
    // 省略
  }
  void toggleFormMode() {
    // 省略
  }

  Widget build(BuildContext context) {
    return Scaffold(
        // 省略
        );
  }

 // 省略
}

homepage.dart

class HomePage extends StatelessWidget {
  HomePage(
      {Key key, this.auth, this.onSignedOut, this.userId, this.logoutCallback})
      : super(key: key);

  final BaseAuth auth;
  final VoidCallback onSignedOut;
  final VoidCallback logoutCallback;
  final String userId;

  Widget build(BuildContext context) {
    //★TODO: 認証情報等をすべて引数で下位Widgetに渡している箇所はProviderでリファクタリングする
    return MaterialApp(
        home: MyHomePage(
            title: 'title',
            auth: this.auth,
            onSignedOut: this.onSignedOut,
            logoutCallback: this.logoutCallback,
            userId: this.userId),
        routes: <String, WidgetBuilder>{
          '/login_signup': (BuildContext context) => LoginSignupPage(),
        });
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage(
      {Key key,
      this.title,
      this.auth,
      this.onSignedOut,
      this.logoutCallback,
      this.userId})
      : super(key: key);

  final String title;
  final BaseAuth auth;
  final VoidCallback logoutCallback;
  final String userId;
  final VoidCallback onSignedOut;

  _MyHomePageState createState() => _MyHomePageState(
      auth: this.auth, onSignedOut: this.onSignedOut, userId: this.userId);
}

class _MyHomePageState extends State<MyHomePage> {
  FirebaseUser user;
  final BaseAuth auth;
  final logoutCallback;
  final VoidCallback onSignedOut;

  _MyHomePageState(
      {this.auth, this.onSignedOut, this.logoutCallback, this.userId});

  void initState() {
    super.initState();

  initUser() async {
    setState(() {});
  }

  final String userId;
  String displayName = '名無し';

  Widget build(BuildContext context) {
    Firestore.instance
        .collection('users')
        .document(userId)
        .get()
        .then((onValue) {
      displayName = onValue.data['displayName'].toString();
    });

    return Scaffold(
      appBar: Header(),
      body: Column(
        children: <Widget>[
          Expanded(
            flex: 0,
            child: Row(
              children: <Widget>[
                // 省略
                Expanded(
                  flex: 1,
                  child: FlatButton(
                    // 省略
                    onPressed: () async {
                      try {
                        //★logoutCallbackが呼べていない？
                        await widget.auth.signOut();
                        widget.logoutCallback;
                      } catch (e) {
                      }
                      //★ここも怪しい
                      Navigator.of(context)
                          .pushReplacementNamed('/login_signup');
                    },
                  ),
                )
              ],
            ),
          ),
              // 省略
        ],
      ),
    );
  }
}