在下面的代码片段中，我如何确保在检查列表包含的内容之前填充senderID列表？我从firestore提取数据，将其存储在列表中，然后检查它是否包含特定的键。

Stream<List<Group>> getChatGroups(String grpId) {
        return firestore.collection('groups').snapshots().map((event) {
          List<Group> groups = [];
          List<String> senderIds = [];
    
          for (var document in event.docs) {
            var group = Group.fromMap(document.data());
    
            firestore.collection('groups').doc(group.groupId).collection('chats').get().then((QuerySnapshot snapshot) {
              snapshot.docs.forEach((DocumentSnapshot doc) {
                var messageData = doc.data() as Map<String, dynamic>;
                var messages = Message.fromMap(messageData);
                var grId = doc.reference.parent.parent?.id;
    
    //The values in SenderIds should be set before the function below is initiaited 
                senderIds.add(messages.senderId);
    
              });
              
            });
    
    
    //This if function should initiate after getting set above
            if (senderIds.contains(auth.currentUser!.uid)) {
              groups.add(group);
            }
          }
          return groups;
        });
      }