无法使用提供程序和StreamBuilder从Firestore加载数据

Question

我无法加载已登录用户上传到Firestore的数据： 我的代码没有错误，我在StreamBuilder中使用提供程序包： 代码为：

我正在使用支架主体中的_buildContents方法，以在主屏幕上显示Firestore数据，

class ReportsPage extends StatelessWidget {

    Future<void> _signOut(BuildContext context) async {
        try {
          final auth = Provider.of<AuthBase>(context);
          await auth.signOut();
        } catch (e) {
          print(e.toString());
        }
    }

    Widget build(BuildContext context) {
        return Scaffold(
          appBar: AppBar(
            title: Text('Report'),
            actions: <Widget>[
              FlatButton(
                child: Text(
                  'Logout',
                  style: TextStyle(fontSize: 18.0, color: Colors.white),
                ),
                onPressed: () => _confirmSignOut(context),
              )
            ],
          ),
          body:_buildContents( context),
          floatingActionButton: FloatingActionButton(
            child: Icon(Icons.add),
            onPressed: () => AddReportPage.show(context),
          ),
        );
    }
}

Widget _buildContents(BuildContext context) {
   final database = Provider.of<Database>(context);
   return StreamBuilder<List<Report>>(
     stream: database.reportsStream(),
       builder: (context,snapshot){
       if(snapshot.hasData){
         final report= snapshot.data;
         final children = report.map((reports) => Text(reports.crimeType))
             .toList();
         return ListView(children: children);
       }
       if(snapshot.hasError){
         return Center(
           child: Text('Some Error Occurred'),
         );
       }
       return Center(
         child: CircularProgressIndicator(),
       );
     },
   );
}

     abstract class Database{
    Stream<List<Report>> reportsStream();
  Future<void> createReport(Report report);

 }
  String documentIdFormCurrentDate() => DateTime.now().toIso8601String();

   class FirestoreDatabase implements Database {
     FirestoreDatabase({@required this.uid}) : assert(uid != null);
     final String uid;

     final _service = FirestoreService.instance;

     Future<void> createReport(Report report) async => _service.setData(
           path: APIPath.report(uid, documentIdFormCurrentDate()),
           data: report.toMap(),
         );

     //TODO: temporary code:
     Stream<List<Report>> reportsStream() => _service.collectionStream(
             path: APIPath.reports(uid),
             builder: (data) => Report.fromMap(data),
         );
}

class FirestoreService {
  FirestoreService._();
  static final instance = FirestoreService._();

  Future<void> setData({String path, Map<String, dynamic> data}) async{
    final reference = Firestore.instance.document(path);
    await reference.setData(data);
  }
  Stream<List<T>> collectionStream<T>({
    @required String path,
    @required T builder(Map<String, dynamic> data),
  }) {
    final reference = Firestore.instance.collection(path);
    final snapshots = reference.snapshots();
    return snapshots.map((snapshot) =>
        snapshot.documents.map((snapshot) => builder(snapshot.data))
            .toList());
  }
}

class Report {
  Report({@required this.crimeType, @required this.description});
  final String crimeType;
  final String description;

  factory Report.fromMap(Map<String, dynamic> data){
    if(data=null){
      return null;
    }
    final String crimeType= data['Crime Type'];
    final String description = data['Description'];
    return Report(
      crimeType: crimeType,
      description: description,
    );
  }

  Map<String, dynamic> toMap(){
    return {
      'CrimeType': crimeType,
      'Description' : description,
    };
  }
}

调试控制台为：表示： enter image description here

堆栈跟踪为：

#0      new Report.fromMap (package:onlinecrimereportingsystem/app/Home/Models/Report.dart:10:8)
#1      FirestoreDatabase.reportsStream.<anonymous closure> (package:onlinecrimereportingsystem/services/database.dart:30:40)
#2      FirestoreService.collectionStream.<anonymous closure>.<anonymous closure> (package:onlinecrimereportingsystem/services/firestore_services.dart:19:53)
#3      MappedListIterable.elementAt (dart:_internal/iterable.dart:417:31)
#4      ListIterator.moveNext (dart:_internal/iterable.dart:343:26)
#5      new List.from (dart:core-patch/array_patch.dart:38:29)
#6      new List.of (dart:core-patch/array_patch.dart:68:17)
#7      ListIterable.toList (dart:_internal/iterable.dart:211:44)
#8      FirestoreService.collectionStream.<anonymous closure> (package:onlinecrimereportingsystem/services/firestore_services.dart:20:14)
#9      _MapStream._handleData (dart:async/stream_pipe.dart:219:31)
#10     _ForwardingStreamSubscription._handleData (dart:async/stream_pipe.dart:157:13)
#11     _rootRunUnary (dart:async/zone.dart:1198:47)
#12     _CustomZone.runUnary (dart:async/zone.dart:1100:19)
#13     _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
#14     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:357:11)
#15     _DelayedData.perform (dart:async/stream_impl.dart:611:14)
#16     _StreamImplEvents.handleNext (dart:async/stream_impl.dart:730:11)
#17     _PendingEvents.schedule.<anonymous closure> (dart:async/stream_impl.dart:687:7)
#18     _rootRun (dart:async/zone.dart:1182:47)
#19     _CustomZone.run (dart:async/zone.dart:1093:19)
#20     _CustomZone.runGuarded (dart:async/zone.dart:997:7)
#21     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1037:23)
#22     _rootRun (dart:async/zone.dart:1190:13)
#23     _CustomZone.run (dart:async/zone.dart:1093:19)
#24     _CustomZone.runGuarded (dart:async/zone.dart:997:7)
#25     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1037:23)
#26     _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
#27     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)