下面是一个简单的代码块，用于检测来自SMS的交易：

FlutterLocalNotificationsPlugin notificationsPlugin =
    FlutterLocalNotificationsPlugin();

onBackgroundMessage(SmsMessage message) async {
  log("Message recieved in background");
  _TransactionState().onMessage(message);
}

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

  @override
  State<Transaction> createState() => _TransactionState();
}

class _TransactionState extends State<Transaction> {
  String _message = "";
  var debit = 0.0;
  var credit = 0.0;
  var last = 0.0;
  var transact = 0.0;
  final Telephony telephony = Telephony.instance;

  @override
  void initState() {
    super.initState();
    initPlatformState();
  }

  onMessage(SmsMessage message) async {
    setState(() {
      bool mesFlag = false;
      bool debFlag = false;
      if (message.body!.contains('debit')) {
        debFlag = true;
      } //check whether debit or credit
      for (var bank in banks.entries) {
        if (message.address!.contains(bank.key)) {
          _message = message.body?.split(
                  ' ')[(debFlag ? bank.value.item2 : bank.value.item1)] ??
              "Error reading transaction!";
          _message = _message.replaceAll(',', ''); //remove comma
          if (_message.contains("Rs.")) {
            _message = _message.substring(3);
          } else if (_message.contains("Rs")) {
            _message = _message.substring(2);
          } // remove Rs. and Rs
          showNotification("Last transaction amount: $_message");
          transact = double.parse(_message);
          mesFlag = true;
          if (debFlag) {
            debit += transact;
            last = -transact;
          } else {
            credit += transact;
            last = transact;
          }
          limit += last;
          if (limit < 0) {
            limit = 0;
            showNotification("You are over-spending!");
          }
          transList.add(TransactionTile(trans: last));
          break;
        }
      }
      if (!mesFlag) _message = ''; //if not a transaction
    });
  }

下面是我在后台收到短信时抛出的异常：

setState() called in constructor: _TransactionState#f0921(lifecycle state: created, no widget, not mounted)
This happens when you call setState() on a State object for a widget that hasn't been inserted into the widget tree yet. It is not necessary to call setState() in the constructor, since the state is already assumed to be dirty when it is initially created.

电话软件包要求onBackgroundMessage是一个全局函数。是否有任何变通办法，通过它我可以调用全局函数中的onMessage或setState()，因为我需要他们被更新，即使当应用程序没有打开&它应该完全像onMessage功能。
任何帮助都感激不尽。