我正在构建一个应用程序，到目前为止一切正常。直到我单击一个调用此Staful小部件的按钮：

    class ToDo1 extends StatefulWidget {
      @override
      _ToDo1State createState() => _ToDo1State();
    }
    
    class _ToDo1State extends State<ToDo1> {
      User? user;
      late DatabaseService database;
    
      void toggleDone(String key, bool value) {
        database.setTodo(key, !value);
      }
    
      Future<void> connectToFirebase() async {
        final FirebaseAuth _auth = FirebaseAuth.instance;
        UserCredential result = await _auth.signInAnonymously();
        user = result.user;
        database = DatabaseService(user!.uid);
    
        if (!(await database.checkIfUserExists())) {
          database.setTodo('To-Do anlegen', false);
        }
      }
    
      @override
      Widget build(BuildContext context) {
        return Scaffold(
            appBar: AppBar(
              title: Center(
                  child: Text(
                'Stufe 1',
                style: TextStyle(
                    fontStyle: FontStyle.italic,
                    decoration: TextDecoration.underline),
              )),
              backgroundColor: Color.fromRGBO(35, 112, 192, 1),
            ),
            body: FutureBuilder(
              future: connectToFirebase(),
              builder: (BuildContext context, AsyncSnapshot<void> snapshot) {
                if (snapshot.connectionState != ConnectionState.done) {
                  return Center(child: CircularProgressIndicator());
                } else {
                  return StreamBuilder<DocumentSnapshot>(
                      stream: database.getTodos(),
                      builder: (context, AsyncSnapshot<DocumentSnapshot> snapshot) {
                        if (!snapshot.hasData) {
                          return Center(child: CircularProgressIndicator());
                        } else {
                          Map<String, dynamic> items =
                              snapshot.data!.data as Map<String, dynamic>;
                          return ListView.separated(
                              separatorBuilder: (BuildContext context, int index) {
                                return SizedBox(
                                  height: 10,
                                );
                              },
                              padding: EdgeInsets.all(10),
                              itemCount: items.length,
                              itemBuilder: (BuildContext, i) {
                                String key = items.keys.elementAt(i);
                                return ToDoItem(
                                  key,
                                  items[key]!,
                                  () => toggleDone(key, items[key]),
                                );
                              });
                        }
                      });
                }
              },
            ));
      }
    }

首先出现加载屏幕，然后我遇到以下错误：

W/DynamiteModule(22637): Local module descriptor class for providerinstaller not found.
I/DynamiteModule(22637): Considering local module providerinstaller:0 and remote module providerinstaller:0
W/ProviderInstaller(22637): Failed to load providerinstaller module: No acceptable module found. Local version is 0 and remote version is 0.
W/om.example.akn(22637): Reducing the number of considered missed Gc histogram windows from 150 to 100
W/om.example.akn(22637): Accessing hidden method Ldalvik/system/CloseGuard;->warnIfOpen()V (greylist,core-platform-api, linking, allowed)

======== Exception caught by widgets library =======================================================
The following _CastError was thrown building StreamBuilder<DocumentSnapshot<Object?>>(dirty, state: _StreamBuilderBaseState<DocumentSnapshot<Object?>, AsyncSnapshot<DocumentSnapshot<Object?>>>#4f0bb):
type '() => Map<String, dynamic>?' is not a subtype of type 'Map<String, dynamic>' in type cast

The relevant error-causing widget was: 
  StreamBuilder<DocumentSnapshot<Object?>> file:///C:/Users/Lars_/StudioProjects/akne/lib/Fl%C3%A4che%203.1/To-Dos.dart:53:22
When the exception was thrown, this was the stack: 
#0      _ToDo1State.build.<anonymous closure>.<anonymous closure> (package:akne/Fl%C3%A4che%203.1/To-Dos.dart:59:72)
#1      StreamBuilder.build (package:flutter/src/widgets/async.dart:546:81)
#2      _StreamBuilderBaseState.build (package:flutter/src/widgets/async.dart:124:48)
#3      StatefulElement.build (package:flutter/src/widgets/framework.dart:4691:27)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4574:15)
...
====================================================================================================

我认为错误发生在这里：

    Map<String, dynamic> items = snapshot.data!.data as Map<String, dynamic>;

这是与Firebase交互的类：

class DatabaseService {

  final String userID;
  DatabaseService(this.userID);

  final CollectionReference userTodos =
  FirebaseFirestore.instance.collection('userTodos');

  Future setTodo(String item, bool value) async {
    return await userTodos.doc(userID).set(
        {item:value}, SetOptions(merge: true));
  }

  Future deleteTodo(String key) async {
    return await userTodos.doc(userID).update(
        {key: FieldValue.delete(),}
    );
  }

  Future checkIfUserExists() async {
    if((await userTodos.doc(userID).get()).exists) {
      return true;
    }
    else {
      return false;
    }
  }

  Stream<DocumentSnapshot> getTodos() {
    return userTodos.doc(userID).snapshots();
  }
}

我希望我已经提供了所有必要的数据，以便解决问题。如果没有，就写信给我，我会试着给你发你需要的材料。