Stateless não está funcionando

Tive exatamente o mesmo problema. Alterei o Widget de Stateless para Stateful e resolvi.

Não funcionou apenas alterar. Tive que fazer isso aqui

import 'package:bytebank/database/dao/contacts_dao.dart';
import 'package:bytebank/models/contacts.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('Contacts'),
      ),
      body: FutureBuilder<List<Contact>>(
        initialData: List(),
        future: _dao.findAllContacts(),
        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...')
                  ],
                ),
              );
              break;
            case ConnectionState.active:
              break;
            case ConnectionState.done:
              debugPrint(snapshot.data.toString());
              final List<Contact> contacts = snapshot.data;
              return ListView.builder(
                itemBuilder: (context, index) {
                  final Contact contact = contacts[index];
                  return _ContactItem(contact);
                },
                itemCount: contacts.length,
              );
              break;
          }
          return Text('Unknown error');
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () async{
          await Navigator.of(context).push(
            MaterialPageRoute(
              builder: (context) => ContactForm(),
            ),
          );
          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,
          ),
        ),
      ),
    );
  }
}