Erro na lista após abstração

Boa noite, após a abstração dos códigos, ao clicar na lista de contatos, o app gera o erro: type 'Null' is not a subtype of type 'List<dynamic>' in type cast

Segue meu arquivo contact_dao.dart

class ContactDao {
  static const String _tableName = 'contacts';
  static const String _id = 'id';
  static const String _name = 'name';
  static const String _accountNumber = 'account_number';

  static const String tableSql = 'CREATE TABLE contacts('
      'id INTEGER PRIMARY KEY, '
      'name TEXT, '
      'accountNumber INTEGER)';

  Future<int> save(Contact contact) async {
    final Database db = await getDatabase();
    Map<String, dynamic> contactMap = _toMap(contact);
    return db.insert(_tableName, contactMap);
  }

  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;
  }

  List<Contact> _toList(List<Map<String, dynamic>> result) {
    final List<Contact> contacts = [];
    for (Map<String, dynamic> row in result) {
      final Contact contact = Contact(
        row[_id],
        row[_name],
        row[_accountNumber],
      );
      contacts.add(contact);
    }
    return contacts;
  }

  Map<String, dynamic> _toMap(Contact contact) {
    final Map<String, dynamic> contactMap = {};
    contactMap[_name] = contact.name;
    contactMap[_accountNumber] = contact.accountNumber;
    return contactMap;
  }
}

Segue o build do contact_list.dart

Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Contacts'),
        backgroundColor: Theme.of(context).primaryColor,
      ),
      body: FutureBuilder(
        initialData: const [],
        future: _dao.findAll(),
        builder: (context, snapshot) {
          switch (snapshot.connectionState) {
            case ConnectionState.waiting:
              return Center(
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  crossAxisAlignment: CrossAxisAlignment.center,
                  children: const <Widget>[
                    CircularProgressIndicator(color: Colors.green),
                    SizedBox(
                      height: 20,
                    ),
                    Text(
                      'Loading',
                      style: TextStyle(fontSize: 25),
                    ),
                  ],
                ),
              );
            case ConnectionState.active:
              // TODO: Handle this case.
              break;
            case ConnectionState.done:
              final contacts = (snapshot.data as List);
              return ListView.builder(
                itemCount: contacts.length,
                itemBuilder: (context, index) {
                  final contact = contacts[index];
                  return _ContactItem(contact);
                },
              );
          }
          return const Text('Unknown error');
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context)
              .push(MaterialPageRoute(builder: (context) => const ContactForm())).then((value) => setState(() {}));
        },
        child: const Icon(Icons.add),
      ),
    );
  }

Antes das abstrações estava funcionando mas tentei voltar e mesmo assim não consegui. Alguma luz amiga...???