meu código esta igual ao dá aula mas não funciona.

class TransactionsList extends StatefulWidget {
  const TransactionsList({Key? key}) : super(key: key);

  @override
  State<TransactionsList> createState() => _TransactionsListState();
}

class _TransactionsListState extends State<TransactionsList> {
  final TransactionWebclient _webclient = TransactionWebclient();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Transactions'),
      ),
      body: FutureBuilder<List<Transaction>>(
        future: _webclient.findAll(),
        builder: (context, snapshot) {
          switch (snapshot.connectionState) {
            case ConnectionState.none:
              break;
            case ConnectionState.waiting:
              return Progress();
            case ConnectionState.active:
              break;
            case ConnectionState.done:
              if (snapshot.hasData) {
               final List<Transaction>? transactions = snapshot.data;
                if (transactions!.isNotEmpty) {
                  return ListView.builder(
                    itemBuilder: (context, index) {
                      final Transaction transaction = transactions[index];
                      return Card(
                        child: ListTile(
                          leading: const Icon(Icons.monetization_on),
                          title: Text(
                            transaction.value.toString(),
                            style: const TextStyle(
                              fontSize: 24.0,
                              fontWeight: FontWeight.bold,
                            ),
                          ),
                          subtitle: Text(
                            transaction.contact.accountNumber.toString(),
                            style: const TextStyle(
                              fontSize: 16.0,
                            ),
                          ),
                        ),
                      );
                    },
                     itemCount: transactions.length,
                  );
                }
              }

              return  CenteredMessage(
                'No transactions found',
                icon: Icons.warning,
              );
          }
          return  CenteredMessage(
            'Unknown error',
            icon: Icons.warning,
          );
        },
      ),
    );
  }
}