Bad notification for startForeground error

I tried to background services like this code, but after first alert app crashed. And gave this error:

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 defaults=0x0 flags=0x40 color=0x00000000 vis=PRIVATE)

My code :

 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                mNotification = Notification.Builder(this, CHANNEL_ID)
                        // Set the intent that will fire when the user taps the notification
                        .setContentIntent(pendingIntent)
                        .setSmallIcon(R.drawable.ic_stat_name)
                        .setLargeIcon(BitmapFactory.decodeResource(res, R.mipmap.ic_launcher))
                        .setAutoCancel(true)
                        .setContentTitle(title)
                        .setStyle(Notification.BigTextStyle()
                                .bigText(message))
                        .setContentText(message).build()

            } else {

                mNotification = Notification.Builder(this)
                        // Set the intent that will fire when the user taps the notification
                        .setContentIntent(pendingIntent)
                        .setSmallIcon(R.drawable.ic_stat_name)
                        .setLargeIcon(BitmapFactory.decodeResource(res, R.mipmap.ic_launcher))
                        .setAutoCancel(true)
                        .setPriority(Notification.PRIORITY_MAX)
                        .setContentTitle(title)
                        .setStyle(Notification.BigTextStyle()
                                .bigText(message))
                        .setContentText(message).build()
            }



            notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager

            notificationManager.notify(mNotificationId, mNotification)

You forgot to create a channel. It should be like this:

mNotification = Notification.Builder(this, CHANNEL_ID)
                        // Set the intent that will fire when the user taps the notification
                        .setContentIntent(pendingIntent)
                        .setSmallIcon(R.drawable.ic_stat_name)
                        .setLargeIcon(BitmapFactory.decodeResource(res, R.mipmap.ic_launcher))
                        .setAutoCancel(true)
                        .setContentTitle(title)
                        .setStyle(Notification.BigTextStyle()
                                .bigText(message))
                        .setContentText(message).build()
notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            val channel = NotificationChannel(CHANNEL_ID,
                    "Channel title",
                    NotificationManager.IMPORTANCE_DEFAULT)
            notificationManager.createNotificationChannel(channel)
        }

            notificationManager.notify(mNotificationId, mNotification)