Firebase – Flutter Unhandled Exception: NoSuchMethodError: The method 'then' was called on null

Exception has occurred.
NoSuchMethodError (NoSuchMethodError: The method ‘then’ was called on null.
Receiver: null
Tried calling: then(Closure: (dynamic) => Null))
Friends, I am getting the error I mentioned above. Does anyone know how to solve this?
I’m trying to user search via firebase.Does anyone know any other way to do this?

    class _DiscoverScreen extends State<DiscoverScreen> {
  DatabaseMethods databaseMethods = DatabaseMethods();
  TextEditingController searchEditingController = TextEditingController();
  QuerySnapshot? searchResultSnapshot;

  @override
  void dispose() {
    searchEditingController.dispose();
    super.dispose();
  }

  initiateSearch() async {
    if (searchEditingController.text.isNotEmpty) {
      await databaseMethods
          .searchByName(searchEditingController.text)
          .then((value) {
        value.docs.forEach(
          (result) {
            searchResultSnapshot = value;
            print("$searchResultSnapshot");
          },
        );
      });
    }
  }

  Widget userList() {
    return searchResultSnapshot != null
        ? ListView.builder(
            shrinkWrap: true,
            itemCount: searchResultSnapshot!.docs.length,
            itemBuilder: (context, index) {
              return SearchTile(
                name: searchResultSnapshot!.docs[index].get("name"),
                username: searchResultSnapshot!.docs[index].get("username"),
                image: searchResultSnapshot!.docs[index].get("photoUrl"),
              );
            })
        : Container();
  }

  @override
  Widget build(BuildContext context) {
    Size size = MediaQuery.of(context).size;
    return Scaffold(
      appBar: AppBar(
        centerTitle: false,
        title: Text("Keşfet"),
        backgroundColor: Colors.grey.shade700,
      ),
      body: Column(
        children: [
          Container(
            padding: EdgeInsets.symmetric(horizontal: 24, vertical: 16),
            child: Row(
              children: [
                Expanded(
                  child: TextField(
                    decoration: InputDecoration(
                      border: InputBorder.none,
                      hintText: "Search username",
                      hintStyle: TextStyle(
                        color: Colors.black,
                        fontSize: 16,
                      ),
                    ),
                    style: TextStyle(
                      color: Colors.black,
                      fontSize: 20,
                    ),
                    controller: searchEditingController,
                  ),
                ),
                IconButton(
                    onPressed: () {
                      initiateSearch();
                    },
                    icon: ImageIcon(
                      AssetImage(
                        "assets/icons/searchicon.png",
                      ),
                      size: 20,
                    ))
              ],
            ),
          ),
          userList(),
        ],
      ),
    );
  }
}

searchByName Codes =

searchByName(String username) {
    FirebaseFirestore.instance
        .collection("users")
        .where("username", isEqualTo: username)
        .get();
  }