listenPrinterConnectState method Null safety

Implementation

@override
Stream<List<PrinterDevice>?> listenPrinterConnectState() {
  StreamSubscription? subscription;
  StreamController<List<PrinterDevice>> controller =
      StreamController(onCancel: () {
    subscription?.cancel();
  });
  subscription =
      _connectStateChannel.receiveBroadcastStream().listen((e) async {
    controller.add(await getConnectedPrinters());
  }, onDone: controller.close, cancelOnError: true);
  return controller.stream;
}