在Firestore中创建记录时，小部件库错误捕获了异常

如何解决在Firestore中创建记录时，小部件库错误捕获了异常

我正在从事在线公告板项目，我使用以下代码上传了通知，但上传工作正常，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],)
         

      ],);

  }
);

}

这是这里的代码，我可以将通知上传为图像，也可以将当前时间上传到Firestore。 firestore记录是通过此代码创建的。但是会出现以下错误。

════════ 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:
════════════════════════════════════════════════════════════════════════════════

解决方法

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，因此首先显示一个加载小部件，然后使用hasData检查是否返回了任何数据