Erro "transaction already exists" não aparece

Boa tarde. Fazendo o teste de adicionar a mesma transação, não aparece o erro na tela que a transação já existe, apenas a mensagem de sucesso. No console apresenta a mensagem esperada em seguida de um erro:

I/flutter (8074): Response
I/flutter ( 8074): status code: 409
I/flutter ( 8074): headers: {content-type: text/plain;charset=UTF-8, date: Sun, 18 Jul 2021 19:18:06 GMT, content-length: 26}
I/flutter ( 8074): body: transaction already exists
E/flutter ( 8074): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Looking up a deactivated widget's ancestor is unsafe.
E/flutter ( 8074): At this point the state of the widget's element tree is no longer stable.
E/flutter ( 8074): To safely refer to a widget's ancestor in its dispose() method, save a reference to the ancestor by calling dependOnInheritedWidgetOfExactType() in the widget's didChangeDependencies() method.

Segue o código:

class TransactionWebClient {
  Future<List<Transaction>> findAll() async {
    final http.Response response = await client
        .get(Uri.parse(baseURL))
        .timeout(const Duration(seconds: 5));

    final List<dynamic> decodedJson = jsonDecode(response.body);
    return decodedJson
        .map((dynamic json) => Transaction.fromJson(json))
        .toList();
  }

  Future<Transaction?> save(Transaction transaction, String password) async {
    final String transactionJson = jsonEncode(transaction.toJson());

    await Future.delayed(Duration(seconds: 10));

    final http.Response response = await client
        .post(
          Uri.parse(baseURL),
          headers: {"Content-type": "application/json", "password": password},
          body: transactionJson,
        )
        .timeout(Duration(seconds: 5));

    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 "Unknown error";
  }

  static final Map<int, String> _statusCodeResponses = {
    400: "there was an error submitting transaction",
    401: "authentication failed",
    409: "transaction already exists",
  };
}

class HttpException implements Exception {
  final String? message;

  HttpException(this.message);
}