Unsupported operation:

Ao executar o código no Flutter 2.2.0 da esse erro.

import 'package:bytebank/dao/contact_dao.dart';
import 'package:bytebank/modelos/contact.dart';
import 'package:bytebank/screens/contact_form.dart';
import 'package:flutter/material.dart';

class ContactsList extends StatefulWidget {
 @override
  _ContactsListState createState() => _ContactsListState();
}

class _ContactsListState extends State<ContactsList> {


 final ContactDao _dao = ContactDao();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Contatos'),
      ),
      body: FutureBuilder<List<Contact>>(
        initialData: const[],
        future: _dao.findAll(),
        builder: (context, snapshot) {
          switch (snapshot.connectionState){
            case ConnectionState.none:
                break;
              case ConnectionState.waiting:
                return Center(
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    crossAxisAlignment: CrossAxisAlignment.center,
                    children: <Widget>[
                      CircularProgressIndicator(),
                      Text('Loading')
                    ],
                  ),
                );
              case ConnectionState.active:
                break;
              case ConnectionState.done:
              // ignore: unused_local_variable
              final List<Contact>? contacts = snapshot.data;
              return  ListView.builder(
                itemBuilder: (context, index) {
          final Contact contact = contacts![index];
          return _ContactItem(contact);
        },
        itemCount: contacts!.length,
      );
        }
        return Text('Unknown Error');
        }),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context).push(
            MaterialPageRoute(
              builder: (context) => ContactForm(),
            ),
          )
          .then((value) => setState((){}),
          );
        },
        child: Icon(Icons.add),
      ),
    );
  }
}


class _ContactItem extends StatelessWidget {

  final Contact contact;
  _ContactItem(this.contact);

  @override
  Widget build(BuildContext context) {
    return Card(
            child: ListTile(
              title: Text(
               contact.name,
                  style: TextStyle(
                    fontSize: 24.0,
                  )),
              subtitle: Text(
                contact.accountNumber.toString(),
                style: TextStyle(fontSize: 16.0),
              ),
    ),
   );
  }
}