如何从Firebase Firestore到列表检索孩子的Collection快照

Question

我想从Firestore子级Collection中检索数据并转换为列表项

final FirebaseAuth _auth = FirebaseAuth.instance;

final CollectionReference kayitCollection =  Firestore.instance.collection("kayit");

// get kayit stream
Stream<List<Kayit>> get kayitlar {
    return kayitCollection.document(uid).collection("listem").snapshots().map(_kayitListFromSnapshot); 
}

// kayit list from snapshot
List<Kayit> _kayitListFromSnapshot(QuerySnapshot snapshot){

    return snapshot.documents.map((doc){

        return Kayit(
            taskName: doc.data['taskname'] ?? 'boş' ,
            isCompleted: doc.data['isCompleted'] ?? false,
        );
    }).toList();
}

此列表返回null，此代码有什么问题？