未处理的异常：NoSuchMethodError：在 null 上调用了 getter 'uid'答案

【问题标题】：Unhandled Exception: NoSuchMethodError: The getter 'uid' was called on null未处理的异常：NoSuchMethodError：在 null 上调用了 getter 'uid'
【发布时间】：2021-01-11 18:27:10
【问题描述】：

当用户登录时，我正在从 appBar 中的 firebase 检索用户名。我成功检索了它。但是当用户未登录时。我收到“uid”为空的错误。我相信错误在我的 initUser() 中。但我似乎无法修复它！

E/flutter (16741): [ERROR:flutter/lib/ui/ui_dart_state.cc(166)] 未处理的异常：NoSuchMethodError：getter 'uid' 被调用为 null。 E/颤振（16741）：接收器：空 E/flutter (16741)：尝试调用：uid

class MainScreen extends StatefulWidget {
  final bool isAdmin;

  const MainScreen({Key key, this.isAdmin}) : super(key: key);

  @override
  _MainScreenState createState() => _MainScreenState(this.isAdmin);
}

class _MainScreenState extends State<MainScreen> {
  bool isPressed = false;
  bool userLoggedIn =false;
  //var _userName;
//  Future<FirebaseUser> getUser() {
//    return _auth.currentUser();
//  }
  Future<String> currentUser() async {
   user = await _auth.currentUser();
    return user != null ? user.uid : null;
  }
  FirebaseUser user;
  @override
  void initState() {
    super.initState();

    initUser();

  }
  String userid;

  initUser() async {
    user = await _auth.currentUser();
    userid = user.uid;
    setState(() {});
  }

  bool isAdmin = true;
  final FirebaseAuth _auth = FirebaseAuth.instance;
  //final authData = snapshot.data;
  _MainScreenState(this.isAdmin);

  @override
  Widget build(BuildContext context) {
    SystemChrome.setPreferredOrientations([
      DeviceOrientation.portraitUp,
    ]);
    CurrentUser _currentUser = Provider.of<CurrentUser>(context, listen: false);
    return Scaffold(
      //backgroundColor: Colors.grey,
      resizeToAvoidBottomPadding: false,
      appBar: AppBar(
        flexibleSpace: Container(
          decoration: BoxDecoration(
            gradient: LinearGradient(
              colors: [Colors.cyan, Colors.yellow], stops: [0.5, 1.0],
            ),
          ),
        ),
        //title: Text('Property Host'),

        centerTitle: true,
        actions: <Widget>[
          Expanded(

            child: Row(
            mainAxisAlignment: MainAxisAlignment.spaceAround,
              children: <Widget>[
                //Image.asset('assets/index.jpg', fit: BoxFit.cover,height:16,width:16),
                Container(
                    margin: new EdgeInsets.only(left: 50),
                    child: Text('Property Host',style: TextStyle(fontWeight: FontWeight.bold,fontSize: 19),)),

              user!=null ? StreamBuilder(stream: Firestore.instance.collection('users').where("uid", isEqualTo: userid).snapshots(),

    // ignore: missing_return
    builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
      if (snapshot.data == null)
        return CircularProgressIndicator();
      //final userDocument = snapshot.data;
      //final title=  snapshot.data.userocument['displayName']);
      //CircularProgressIndicator();
      return Expanded(
        child: ListView.builder(
            itemCount: snapshot.data.documents.length,
            // ignore: missing_return
            itemBuilder: (BuildContext context, int index) {
              print(user.uid);
              return user != null
                  ? Container(
                margin: EdgeInsets.only(top: 17, left: 40),
                child: Text(
                    snapshot.data.documents.elementAt(index)['displayName']),
              )
                  : IconButton(
                icon: Icon(Icons.person),
                // ignore: missing_return
                onPressed: () {
                  Navigator.pushNamed(context, '/LoginScreen');
                },
              );
            }
        ),
      );
    }

【问题讨论】：

答案就在您的问题中：“但是当用户未登录时。我收到错误提示 'uid' 为空。”

标签： firebase flutter firebase-authentication

【解决方案1】：

当用户未登录时，await _auth.currentUser() 将返回null。这意味着在user（为空）上调用uid 将导致错误。

您可以将它包裹在 if 语句中的空检查。

initUser() async {
  user = await _auth.currentUser();
  if (user != null) {
    userid = user.uid;
  } else {
    // User is not available. Do something else
  }
  setState(() {});
}

【讨论】：