when trying to implement DialogFlowtter in flutter , I get error for .fromFile()

Question

here is my code.

class _ChatbotScreenState extends State<ChatbotScreen> {
  DialogFlowtter? dialogFlowtter;
  final TextEditingController _controller = TextEditingController();
  List<Map<String, dynamic>> messages = [];

  @override
  void initState() {
    DialogFlowtter.fromFile().then((instance) => dialogFlowtter = instance);  // here
    super.initState();
  }
  @override
  Widget build(BuildContext context) {
 .....

the error is, The method 'fromFile' isn't defined for the type 'DialogFlowtter'. Try correcting the name to the name of an existing method, or defining a method named 'fromFile'.

Added the json file to assets.

I added, the package 'dialog_flowtter: ' and import 'package:dialog_flowtter/dialog_flowtter.dart';