type 'Null' is not a subtype of type 'Future<List<Contact>>'

Estou com esse problema mesmo depois de aplicar a solução para gerar um mock, utilizando o @GenerateMocks, eu li essa solução em uma das duvidas aqui do curso, e tentei implementar, porém continuo tendo o mesmo problema, já consultei o repositório para ver se tinha feito algo errado, mas está tudo de acordo com oque foi passado, como eu posso resolver?

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following _TypeError was thrown running a test:
type 'Null' is not a subtype of type 'Future<List<Contact>>'

When the exception was thrown, this was the stack:
#0      MockContactDao.findAll (package:bytebank/database/dao/contact_dao.dart:26:25)
#1      main.<anonymous closure> (file:///D:/Projetos/AluraFlutter/bytebank/test/contact_list_flow_test.dart:17:25)
#2      main.<anonymous closure> (file:///D:/Projetos/AluraFlutter/bytebank/test/contact_list_flow_test.dart:15:40)
#3      testWidgets.<anonymous closure>.<anonymous closure> (package:flutter_test/src/widget_tester.dart:170:29)
<asynchronous suspension>
<asynchronous suspension>
(elided one frame from package:stack_trace)

The test description was:
  Should save a contact

linha 26:25 no qual o erro se refere :

  Future<List<Contact>> findAll() async {
    final Database db = await getDatabase();
    final List<Map<String, dynamic>> result = await db.query(_tableName);
    List<Contact> contacts = _toList(result);
    return contacts;
  }

Classe de contact_list:

import 'package:bytebank/database/dao/contact_dao.dart';
import 'package:bytebank/models/contacts.dart';
import 'package:bytebank/screens/contact_form.dart';
import 'package:bytebank/screens/transaction_form.dart';
import 'package:bytebank/widget/progress.dart';
import 'package:flutter/material.dart';

class ContactsList extends StatefulWidget {
  final ContactDao contactDao;
  const ContactsList({Key? key, required this.contactDao}) : super(key: key);

  @override
  State<ContactsList> createState() => _ContactsListState();
}

class _ContactsListState extends State<ContactsList> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).primaryColor,
        title: const Text('Transfers'),
      ),
      body: FutureBuilder<List<Contact>>(
        initialData: const [],
        future: widget.contactDao.findAll(),
        builder: (context, snapshot) {
          switch (snapshot.connectionState) {
            case ConnectionState.none:
              break;
            case ConnectionState.waiting:
              return const Progress();
            case ConnectionState.active:
              break;
            case ConnectionState.done:
              final List<Contact> contacts = snapshot.data!;
              return ListView.builder(
                itemBuilder: (context, index) {
                  Contact contact = contacts[index];
                  return _ContactItem(
                    contact: contact,
                    onClick: () {
                      Navigator.of(context).push(MaterialPageRoute(
                          builder: (context) => TransactionForm(contact)));
                    },
                  );
                },
                itemCount: contacts.length,
              );
          }
          return const Text('Unknown error');
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context)
              .push(MaterialPageRoute(
                  builder: (context) => ContactForm(
                        contactDao: widget.contactDao,
                      )))
              .then((value) {
            setState(() {});
          });
        },
        child: const Icon(Icons.add),
      ),
    );
  }
}