when invoke service.startforegorund method i got error

[BatuhanAkar]

        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {

            int audioFocusRequestResult = audioManager.requestAudioFocus(audioFocusRequest) ;

                if (audioFocusRequestResult == AudioManager.AUDIOFOCUS_REQUEST_GRANTED){


                    getApplicationContext().startService(new Intent(getApplicationContext(),MediaPlaybackService.class)) ;
                    mediaSession.setActive(true) ;

                    mediaPlayer.play();
                    registerReceiver(receiver , intentFilter) ;
                   //  service.startForeground(101 , notification);

                    **service.startForeground(101 , new android.app.Notification());**


                }
        }

i can't see any initialize of the service here

https://developer.android.com/guide/topics/media-apps/audio-app/mediasession-callbacks

---error---

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.service.media.MediaBrowserService.startForeground(int, android.app.Notification)' on a null object reference
at com.batuscode.mediabrowserservice.MediaPlaybackService$SessionCallBack.onPlay(MediaPlaybackService.java:170)
at android.support.v4.media.session.MediaSessionCompat$Callback$MediaSessionCallbackApi21.onPlay(MediaSessionCompat.java:1621)
at android.media.session.MediaSession$CallbackMessageHandler.handleMessage(MediaSession.java:1607)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loopOnce(Looper.java:201)
at android.os.Looper.loop(Looper.java:288)
at android.app.ActivityThread.main(ActivityThread.java:7839)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)