Difference between Delegates.notNull and lateinit Kotlin

I am very confused both looks and works pretty similar. Which one should I go for?

private var mMediaController by Delegates.notNull<MediaControllerCompat>()

or

lateinit private var mMediaController: MediaControllerCompat

Usage:

    @Subscribe
    fun connectToSession(token: MediaSessionCompat.Token) {
         mMediaController = MediaControllerCompat(activity, token)
         mMediaController.registerCallback(mMediaControllerCallback)
    }