RemoteServiceException: Bad notification for startForeground in Android 8.0

I am trying to start startForeground service when app is in background but I am getting this error.

notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        System.out.println("ChannelID: Sip Service:000:"+nChannel);
        try {
            notificationManager.createNotificationChannel(nChannel);
        }catch (Exception e ){
            e.printStackTrace();
        }
    }

When I try to create notification channel then app is crash.

android.app.RemoteServiceException: Bad notification for startForeground: java.lang.RuntimeException: invalid channel for service notification: Notification(channel=null pri=0 contentView=null vibrate=null sound=null smartAlertCount=0x0 defaults=0x0 flags=0x40 color=0x00000000 vis=PRIVATE) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1768) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6501) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)