我已经实现了onesignal在其文档中提到的notificationServiceExtension，但现在我无法在一个信号中运行后台通知处理程序。我已经在main activity中实现了以下代码：：：

package com.example.just_normal

import com.onesignal.OneSignal
import io.flutter.embedding.android.FlutterActivity
import io.flutter.embedding.engine.FlutterEngine
import io.flutter.plugin.common.MethodChannel

private val methodchannel = "background/notification";

class MainActivity: FlutterActivity() {
    
    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
        super.configureFlutterEngine(flutterEngine)
        OneSignal.OSRemoteNotificationReceivedHandler { context, osNotificationReceivedEvent ->
             val CHANNEL : MethodChannel = MethodChannel(flutterEngine.dartExecutor.binaryMessenger , methodchannel)
            CHANNEL.invokeMethod("onBackgroundNotification" , osNotificationReceivedEvent);
            // osNotificationReceivedEvent.complete(null);
        }
   }

}

在上面的代码中，我试图在后台收到通知后立即调用flutter方法。我如何在后台收到通知后立即运行flutter方法。请帮助...