Why the value of a variable does not change when using initState in flutter

Question

I have been trying to obtain the value of groupfav that is stored in FireStore to pass it as part of the reference to a Stream, but even when in initState I obtain the corresponding value, the original variable is not changed to the new value, therefore it does not load the list corresponding to the value obtained, I tried many solutions and ideas, but I can't make the String groupfav change to the new value.

static String? userID = FirebaseAuth.instance.currentUser?.uid;
  static final userColeccion =
      FirebaseFirestore.instance.collection("users");

  String groupfav = ' '; //this value should be replaced by the future value obtained
  late Stream<QuerySnapshot> task;

  @override
  void initState() {
    super.initState();

    userColeccion.doc("$userID").get().then((value) {
      groupfav = value.data()!["groupfav"];
      print(groupfav); //correctly prints the value I want (is "groupid4")
      return groupfav;
    });

    taskGroup = FirebaseFirestore.instance
        .collection("groups")
        .doc(groupfav) // pass the obtained value
        .collection("task")
        .snapshots();
  }

doing all this should replace String groupfav = ' '; to String groupfav = 'groupid4', but it doesn't work, it stays at ' ', thanks!