Flutter Isolate Issue with Platform Channel code

I tried to clear my app notification count using the platform code. And I try to implement isolate for this function in dart side.

I got the above error message to both compute and isolate ways. Both are failed.

Error

Exception: Null check operator used on a null value
MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:142:86)
MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:148:36)
MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:331:12)
NotificationCountService.computeFunction (package:yellow/services/notificationCountService.dart:32:16)
_IsolateConfiguration.apply (package:flutter/src/foundation/_isolates_io.dart:81:34)
_spawn.<anonymous closure> (package:flutter/src/foundation/_isolates_io.dart:88:65)
_spawn.<anonymous closure> (package:flutter/src/foundation/_isolates_io.dart:87:5)
Timeline.timeSync (dart:developer/timeline.dart:163:22)
_spawn (package:flutter/src/foundation/_isolates_io.dart:85:35)
_delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:286:17)
_RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)

Full Code

class NotificationCountService {
  static const platform = MethodChannel('path/notificationCount');

  static Future<void> setCountCompute(int count) async {
      //await compute(computeFunction, count);
      spawnNewIsolate(count);
  }

  static computeFunction(int count) async {
    try {
      count = (count ?? 0) <= 0 ? 0 : count;
      compute(await platform.invokeMethod('setCount', {'count': count}), );
      print('result is $result');
    } on PlatformException catch (e) {
      print("Failed to get battery level: '${e.message}'.");
      return null;
    }
  }

  static void spawnNewIsolate(int count) async {
    ReceivePort receivePort = ReceivePort();

    try {
      await Isolate.spawn(createFunction, receivePort.sendPort);

      SendPort childSendPort = await receivePort.first;

      ReceivePort responsePort = ReceivePort();

      childSendPort.send([count, receivePort.sendPort]);

      var response = await responsePort.first;

      print('response $response');

    } catch (e) {
      print("Error: $e");
    }
  }

  static void createFunction(SendPort mainPort) async {
    ReceivePort childPort = ReceivePort();
    mainPort.send(childPort.sendPort);

    await for (var message in childPort) {
      int count = message[0];
      SendPort replyPort = message[1];
      var result = await platform.invokeMethod('setCount', {'count': count});
      replyPort.send(result);
    }
  }
}