在 firestore 中创建记录时，小部件库错误捕获了异常

Question

我正在从事在线布告栏项目，我使用以下代码上传通知，上传工作正常，但出现以下错误。

 class UploadNotice extends StatefulWidget {
   @override
_UploadNoticeState createState() => _UploadNoticeState();
}

class _UploadNoticeState extends State<UploadNotice> {
 final _formKey=GlobalKey<FormState>();
 final List<String> noticrcategory= 
    ['Exams','Mahapola/Bursary','TimeTables','Results','Other','General'];
      File _noticepic;
      String title; 
      String url;
      String category;
      String dateTime;
      var uuid=Uuid();
      bool loading = false;
      DateTime now=new DateTime.now();





      @override
      Widget build(BuildContext context) {
      Future getImage() async{
      var image=await ImagePicker.pickImage(source: ImageSource.gallery);
      setState(() {
          _noticepic=image;
        print('image path: $_noticepic');
       });
      }
      Future uploadPic(BuildContext context)async{
  String fileName=basename(_noticepic.path);
  final StorageReference firebaseStorageRef=FirebaseStorage()
  .ref().child('notices/$fileName');
  final StorageUploadTask uploadTask=firebaseStorageRef.putFile(_noticepic);
  
 StorageTaskSnapshot taskSnapshot= await uploadTask.onComplete;

 String downloadurl = await taskSnapshot.ref.getDownloadURL();
 url=downloadurl.toString();
}
final user = Provider.of<User>(context);

 return StreamBuilder(
  stream:UserService(uid: user.uid).userData,
  builder: (context,snapshot){
    User userData=snapshot.data;
   String getDepartmentName(){
    return userData.department.toString();  
    }
    String department=getDepartmentName();
    
return loading ? Loading(): Scaffold(
    appBar: AppBar(
     elevation: 0.0,
     title: Text('Notices App',
     style: TextStyle(
       fontFamily: 'Montserrat',
       fontWeight: FontWeight.bold,
       color: Colors.white,
     ),
     ),
     backgroundColor: Colors.blue[800],
     

   ),

   body:SingleChildScrollView(
          child: Padding(
            padding: const EdgeInsets.all(8.0),
            child: Form(
              
              key: _formKey,
                  child: Column(
        
      children: <Widget>[
                
                    Padding(
                      padding: const EdgeInsets.all(10.0),
                      child: Container(child: Text('Add your notice here')
                      ),
                    ),
                    Container(
                    child:(_noticepic!=null)?Image.file(_noticepic,fit: BoxFit.fill):
                    Image.asset('',),  
                    height: 450.0,
                    width:300.0,
                    color: Colors.grey[400],
                    
                    ),
                       Padding(
                         padding: const EdgeInsets.only(left: 280),
                         child: Container(
                            
                            child: IconButton(
                              icon: Icon(Icons.add_photo_alternate),
                              color: Colors.grey[700],
                              iconSize: 40,
                               onPressed:(){
                                 getImage().then((context){
                                   uploadPic(context);
                                 });

                               }
                               ),
                          ),
                       ), 
              TextFormField(
                        decoration: new InputDecoration(labelText: 'Title'),
                        
                        validator: (value) {
                          return value.isEmpty ? 'Title is Required' : null;
                        },
                        onChanged: (value) {
                         setState(() => title = value);
                        }),
                        
                 
              DropdownButtonFormField(
                    value: category ?? 'General',
                    items: noticrcategory.map((noticrcategory){
                      return DropdownMenuItem(
                        value: noticrcategory,
                        child: Text('$noticrcategory Category'),
                      );
                    }).toList(),
                    onChanged: (value)=>setState(()=>category=value),

                  ),
                   Text('${now.day}/${now.month}/${now.year}'),
                  SizedBox(height: 30.0,),
                
                Container(
                  height: 30.0,
                  width: 100.0,
                  child: RaisedButton(
                    
                    onPressed: () async {
                    setState(() => loading=true);
                    await NoticeService().updteNoticeData(
                      title,
                      url, 
                      category,
                      'unapproved',     
                       now,
                     '$department',
                      uuid.v4());
                    Navigator.of(context).pushReplacementNamed('/Upload Notices');
                    },
                    
                    child: Text('Upload',style: TextStyle(color: Colors.white),),
                    color: Colors.blue[700],
                  ),
                )
         

      ],
      ),
            ),
          ),
  ),
  

  
  
);

  }
);

}

这里是代码，我可以上传图片形式的通知，我可以将当前时间上传到 firestore。 firestore 记录由此代码创建。但它给出了以下错误。

════════ Exception caught by widgets library ═══════════════════════════════════
The getter 'department' was called on null.
Receiver: null
Tried calling: department
The relevant error-causing widget was
    StreamBuilder<User> 
lib\…\pages\uploadNotice.dart:56
════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by image resource service ════════════════════════════
Unable to load asset:
════════════════════════════════════════════════════════════════════════════════

Answer 1

userData 为空，您应该执行以下操作：

  builder: (context,snapshot){
    if(snapshot.hasData){
     User userData = snapshot.data;
     userData.department.toString(); 
     return Text(userData.department.toString());
    else if (snapshot.hasError) {
      return Text("${snapshot.error}");
    }
    // By default, show a loading spinner.
    return CircularProgressIndicator();
   }

由于 StreamBuilder 是异步的，所以首先显示一个加载小部件并使用 hasData 检查是否返回任何数据

在 firestore 中创建记录时，小部件库错误捕获了异常

I got exception caught by widget library error when creating record in firestore

dart

firebase

flutter

google-cloud-firestore

imagepicker