Flutter Cupertino bottom_tab_bar: 'The method 'method' was called on null'

Question

I have a cupertino bottom_tab_bar on my app, but every time I tap one of the icons to change the page, the debug console shows:

════════ Exception caught by gesture ═══════════════════════════════════════════

The method 'jumpToPage' was called on null.

Receiver: null

Tried calling: jumpToPage(2)

════════════════════════════════════════════════════════════════════════════════

and it does not change the page.

My Dart code:

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:fluttershare/pages/activity_feed.dart';
import 'package:fluttershare/pages/profile.dart';
import 'package:fluttershare/pages/search.dart';
import 'package:fluttershare/pages/timeline.dart';
import 'package:fluttershare/pages/upload.dart';
import 'package:google_sign_in/google_sign_in.dart';

final GoogleSignIn googleSignIn = GoogleSignIn();

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  bool isAuth = false;
  PageController pageController;
  int pageIndex = 0;

  @override
  void initState() {
    super.initState();
    pageController = PageController();
    // Detects when user signed in
    googleSignIn.onCurrentUserChanged.listen((account) {
      handleSignIn(account);
    }, onError: (err) {
      print('Error signing in: $err');
    });
    // Reauthenticate user when app is opened
    googleSignIn.signInSilently(suppressErrors: false).then((account) {
      handleSignIn(account);
    }).catchError((err) {
      print('Error signing in: $err');
    });
  }

  handleSignIn(GoogleSignInAccount account) {
    if (account != null) {
      print('User signed in!: $account');
      setState(() {
        isAuth = true;
      });
    } else {
      setState(() {
        isAuth = false;
      });
    }
  }

  @override
  void dispose() {
    pageController.dispose();
    super.dispose();
  }

  login() {
    googleSignIn.signIn();
  }

  logout() {
    googleSignIn.signOut();
  }

  onPageChanged(int pageIndex) {
    setState(() {
      this.pageIndex = pageIndex;
    });
  }

  onTap(int pageIndex) {
    pageController.jumpToPage(
      pageIndex,
    );
  }

  Scaffold buildAuthScreen() {
    return Scaffold(
      body: PageView(
        children: <Widget>[
          Timeline(),
          ActivityFeed(),
          Upload(),
          Search(),
          Profile(),
        ],
        controller: pageController,
        onPageChanged: onPageChanged,
        physics: NeverScrollableScrollPhysics(),
      ),
      bottomNavigationBar: CupertinoTabBar(
          currentIndex: pageIndex,
          onTap: onTap,
          activeColor: Theme.of(context).primaryColor,
          items: [
            BottomNavigationBarItem(icon: Icon(Icons.whatshot)),
            BottomNavigationBarItem(icon: Icon(Icons.notifications_active)),
            BottomNavigationBarItem(
              icon: Icon(
                Icons.photo_camera,
                size: 35.0,
              ),
            ),
            BottomNavigationBarItem(icon: Icon(Icons.search)),
            BottomNavigationBarItem(icon: Icon(Icons.account_circle)),
          ]),
    );
    // return RaisedButton(
    //   child: Text('Logout'),
    //   onPressed: logout,
    // );
  }

  Scaffold buildUnAuthScreen() {
    return Scaffold(
      body: Container(
        decoration: BoxDecoration(
          gradient: LinearGradient(
            begin: Alignment.topRight,
            end: Alignment.bottomLeft,
            colors: [
              Theme.of(context).accentColor,
              Theme.of(context).primaryColor,
            ],
          ),
        ),
        alignment: Alignment.center,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: <Widget>[
            Text(
              'FlutterShare',
              style: TextStyle(
                fontFamily: "Signatra",
                fontSize: 90.0,
                color: Colors.white,
              ),
            ),
            GestureDetector(
              onTap: login,
              child: Container(
                width: 260.0,
                height: 60.0,
                decoration: BoxDecoration(
                  image: DecorationImage(
                    image: AssetImage(
                      'assets/images/google_signin_button.png',
                    ),
                    fit: BoxFit.cover,
                  ),
                ),
              ),
            )
          ],
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return isAuth ? buildAuthScreen() : buildUnAuthScreen();
  }
}

Flutter version: Already up to date. Flutter is already up to date on channel beta Flutter 1.14.6 • channel beta • https://github.com/flutter/flutter.git Framework • revision fabeb2a16f (4 weeks ago) • 2020-01-28 07:56:51 -0800 Engine • revision c4229bfbba Tools • Dart 2.8.0 (build 2.8.0-dev.5.0 fc3af737c7)

Answer 1

First of all, I would suggest not to make a method named onTap() to maintain good practice.

In your case, in your case, you aren't specifying or providing the pageIndex value when you are calling the method in your onTap property of the CupertinoTabBar Widget. This should be the correct way and should also solve the issue

      bottomNavigationBar: CupertinoTabBar(
          currentIndex: pageIndex,
          onTap: onTap(pageIndex),
    //rest of your code

since you aren't providing a pageIndex, it takes the value as null and tries to call the jumpToPage method on a null value.