Flutter - issues with background task

Question

In my Flutter application, I'm trying to crop image in background using compute() method, because if I try to perform this task on main thread, UI will freeze for many seconds.

This is my method to process image:

Future<Uint8List> processImage(Uint8List bytes) async {
img_lib.Image? image = img_lib.decodeImage(bytes);
if (image != null) {
  double targetAspectRatio = 2 / 3;

  int cropWidth = image.width;
  int cropHeight = (image.width * targetAspectRatio).toInt();

  if (cropHeight > image.height) {
    cropHeight = image.height;
    cropWidth = image.height ~/ targetAspectRatio;
  }

  int cropX = (image.width - cropWidth) ~/ 2;
  int cropY = (image.height - cropHeight) ~/ 2;

  img_lib.Image croppedImage = img_lib.copyCrop(
      image,
      x: cropX,
      y: cropY,
      width: cropWidth,
      height: cropHeight
  );

  List<int> compressedBytes = img_lib.encodeJpg(croppedImage, quality: 60);
  bytes = Uint8List.fromList(compressedBytes);
}
return bytes;}

This is my method to select image and call method processImage() in compute():

Future<void> _selectMedia() async {
final picker = ImagePicker();
final pickedFile = await picker.pickMedia();

  Uint8List bytes = await pickedFile.readAsBytes();
  bytes = await compute(processImage, bytes);

}

Now, when method _selectMedia() is called on button click, I got this error:

E/flutter (17096): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Invalid argument(s): Illegal argument in isolate message: object is unsendable - Library:'dart:async' Class: _AsyncCompleter@4048458 (see restrictions listed at SendPort.send() documentation for more information)

Do someone know what is the problem, and how can I fix it?