Nao consigo retornar o valor da lista de transferencias

class TransactionWebClient {
  Future<List<Transaction>> findAll() async {
    final Response response = await client.get(baseUrl);
    final List<dynamic> decodedJson = jsonDecode(response.body);
    var valor =
        decodedJson.map((dynamic json) => Transaction.fromJson(json)).toList();
    return valor;
  }

  Future<Transaction> save(Transaction transaction, String password) async {
    final String transactionJson = jsonEncode(transaction.toJson());
    await Future.delayed(Duration(seconds: 2));

    final Response response = await client.post(baseUrl,
        headers: {
          'Content-type': 'application/json',
          'password': password,
        },
        body: transactionJson);

    if (response.statusCode == 200) {
      return Transaction.fromJson(jsonDecode(response.body));
    }
    throw HttpException(_getMessage(response.statusCode));
  }

  String? _getMessage(int statusCode) {
    if (_statusCodeResponses.containsKey(statusCode)) {
      return _statusCodeResponses[statusCode];
    }
    return 'Unknow error';
  }

  static final Map<int, String> _statusCodeResponses = {
    400: 'Ocorreu um erro enviando a transferencia',
    401: 'Erro ao autenticar',
    404: 'page not found',
    409: 'Transaction already exists.'
  };
}

class HttpException implements Exception {
  final String? message;

  HttpException(this.message);
}

import 'contact.dart';

class Transaction {
  final String id;
  final double value;
  final Contact contact;

  Transaction(
    this.id,
    this.value,
    this.contact,
  );

  @override
  String toString() {
    return 'Transaction{value: $value, contact: $contact}';
  }

  Transaction.fromJson(Map<String, dynamic> json)
      : id = json['id'],
        value = json['value'],
        contact = Contact.fromJson(json['contact']);

  Map<String, dynamic> toJson() => {
        'id': id,
        'value': value,
        'contact': contact.toJson(),
      };
}

class TransactionForm extends StatefulWidget {
  final Contact contact;

  TransactionForm(this.contact);

  @override
  _TransactionFormState createState() => _TransactionFormState();
}

class _TransactionFormState extends State<TransactionForm> {
  final String transactionId = Uuid().v4();
  final TextEditingController _valueController = TextEditingController();
  final TransactionWebClient _webClient = TransactionWebClient();

  bool _sending = false;

  @override
  Widget build(BuildContext context) {
    print('Transactionform ID = $transactionId');
    return Scaffold(
      appBar: AppBar(
        title: Text('New transaction'),
      ),
      body: SingleChildScrollView(
        child: Padding(
          padding: const EdgeInsets.all(16.0),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              Visibility(
                child: Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: Progress(
                    message: 'Enviando ...',
                  ),
                ),
                visible: _sending,
              ),
              Text(
                widget.contact.name,
                style: TextStyle(
                  fontSize: 24.0,
                ),
              ),
              Padding(
                padding: const EdgeInsets.only(top: 16.0),
                child: Text(
                  widget.contact.accountNumber.toString(),
                  style: TextStyle(
                    fontSize: 32.0,
                    fontWeight: FontWeight.bold,
                  ),
                ),
              ),
              Padding(
                padding: const EdgeInsets.only(top: 16.0),
                child: TextField(
                  controller: _valueController,
                  style: TextStyle(fontSize: 24.0),
                  decoration: InputDecoration(labelText: 'Value'),
                  keyboardType: TextInputType.numberWithOptions(decimal: true),
                ),
              ),
              Padding(
                padding: const EdgeInsets.only(top: 16.0),
                child: SizedBox(
                  width: double.maxFinite,
                  child: ElevatedButton(
                    child: Text('Transfer'),
                    onPressed: () {
                      final double? value =
                          double.tryParse(_valueController.text);
                      final transactionCreated =
                          Transaction(transactionId, value!, widget.contact);
                      showDialog(
                          context: context,
                          builder: (contextDialog) {
                            return TransactionAuthDialog(
                                onConfirm: (String password) {
                              _save(transactionCreated, password.toString(),
                                  context);
                            });
                          });
                    },
                  ),
                ),
              )
            ],
          ),
        ),
      ),
    );
  }

  void _save(Transaction transactionCreated, String password,
      BuildContext context) async {
    //await Future.delayed(Duration(seconds: 1));
    final Transaction transaction = await _send(
      transactionCreated,
      password,
      context,
    );

    _showSuccessfulMessage(transaction, context);
  }

  Future<Transaction> _send(Transaction transactionCreated, String password,
      BuildContext context) async {
    final Transaction transaction =
        await _webClient.save(transactionCreated, password).catchError((e) {
      _showFailureMessage(context, message: e.message);
    }, test: (e) => e is HttpException).catchError((e) {
      _showFailureMessage(context, message: 'timeout');
    }, test: (e) => e is TimeoutException).catchError((e) {
      _showFailureMessage(context);
    }).whenComplete(() {
      setState(() {
        _sending = false;
      });
    });

    return transaction;
  }

  void _showFailureMessage(BuildContext context,
      {String message = 'erro desconhecido'}) {
    showDialog(
        context: context,
        builder: (contextDialog) {
          return FailureDialog(message);
        });
  }

  Future _showSuccessfulMessage(
      Transaction transaction, BuildContext context) async {
    if (transaction != null) {
      await showDialog(
          context: context,
          builder: (contextDialog) {
            return SuccessDialog('sucessful');
          }).then((value) => Navigator.pop(context));
      //Navigator.pop(context);
    }
  }
}

Devido ao Null Safety, na classe Contact no fromJson devemos usar o operador ?? para atribuir um int ao "id" caso o mesmo seja nulo.

Ex: Contact.fromJson(Map<String, dynamic> json) : id = json['id'] ?? 0, name = json['name'] as String, accountNumber = json['accountNumber'] as int;

espero que ajude.