Notification Channel set in AndroidManifest.xml has not been created by the app. Default value will be used

I facing this error even though I had created my own according to this website Creating Notification Channels in Flutter (Android)

Notification Channel requested (myapp.com/notification) has not been created by the app. Manifest configuration, or default, value will be used.

Notification Channel set in AndroidManifest.xml has not been created by the app. Default value will be used.

In the same directory as the MainActivity.kt, I also created another file name Application.kt

package com.myapp

import io.flutter.app.FlutterApplication
import io.flutter.plugin.common.PluginRegistry
import io.flutter.plugin.common.PluginRegistry.PluginRegistrantCallback
import io.flutter.plugins.GeneratedPluginRegistrant
import io.flutter.plugins.firebasemessaging.FlutterFirebaseMessagingService

class Application : FlutterApplication(), PluginRegistrantCallback {

    override fun onCreate() {
        super.onCreate()
        FlutterFirebaseMessagingService.setPluginRegistrant(this);
    }

    override fun registerWith(registry: PluginRegistry?) {
        io.flutter.plugins.firebasemessaging.FirebaseMessagingPlugin.registerWith(registry?.registrarFor("io.flutter.plugins.firebasemessaging.FirebaseMessagingPlugin"));
    }
}

Can anyone show me where is the problems and the solution?