Flutter Map is giving empty Map {}

Question

I am creating a stream of a Dart Data Class by accessing data from Firebase Firestore. Inside a block it is showing correct output but outside that block it is printing empty Map. I declared the map outside that block. Please refer the code snippet..

Stream<Map<UserModel?, double>> getFriendStream() {
    UserModel user;
    return firestore
        .collection('users')
        .doc(auth.currentUser?.phoneNumber)
        .snapshots()
        .asyncMap((event) {
      Map<UserModel?, double> friendObjectList = new Map();

      if (event.data() != null) {
        user = UserModel.fromMap(event.data()!);
        user.friendList.forEach((key, value) async {
          UserModel? doc = await getUserData(key);
          // print(key);
          // print(doc!.name);
          friendObjectList[doc] = value;
          // print(friendObjectList); Here it prints Map correctly
        });
      }
      print(friendObjectList); // printing empty Map {}.
      return friendObjectList;
    });
  }

I tried to add some data to a map. I declared it in higher scope and added items in lower space. and return it at higher scope. In higher scope, when I am printing it is giving Empty map but in lower scope, it gives correct Map.