[^3.0.0.beta4] not working setBackgroundHandler on iOS

[ChangJoo-Park]

just make backgroundHandler top-level method on top of main.dart.

void backgroundHandler() {
  print('background handler');
}

and register handler with setBackgroundHandler

FlutterUploader().setBackgroundHandler(backgroundHandler);

when I start upload, upload is well works but background handler does not print on debug console

[yjt1216]

CocoaPods could not find compatible versions for pod "flutter_uploader":