Implementing Background notification handler on flutter one signal sdk

Question

I am trying to implement the background notification handler in flutter for the one signal flutter sdk. What I am trying to do is call a method in flutter side when a background notification arrives like in the OneSignal docs suggests https://documentation.onesignal.com/docs/service-extensions#android-notification-extender-service . This is my MainActrivity.kt

import androidx.annotation.NonNull
import io.flutter.embedding.engine.FlutterEngine
import io.flutter.plugin.common.MethodChannel
import com.onesignal.OneSignal.OSRemoteNotificationReceivedHandler;

var channel: MethodChannel? = null

class MainActivity: FlutterActivity() {
    private val CHANNEL = "samples.flutter.dev/notification";
    override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine){
        super.configureFlutterEngine(flutterEngine);
        channel = MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL);
    }
}

public class NotificationServiceExtension : OSRemoteNotificationReceivedHandler{
    override fun remoteNotificationReceived(Context context, OSNotificationReceivedEvent notificationReceivedEvent) {
       channel.invokeMethod("testNotification");
    }
}

I have never written Kotlin code before and maybe I am doing something wrong here as this is resulting in build errors. Has anyone here done something similar before? Would really appreciate any help. Thanks in advance!

onesignal_flutter: 3.2.8
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