Empty Timeline despite adding ConcatenatingMediaSouce in ExoPlayer

I am having a bunch of songs of which I am making a ConcatenatingMediaSource

...

allSongs.forEach {
            concatenatingMediaSource.addMediaSource(makeMediaSource(it.source))
        }

...

MakeMediaSource function

private fun makeMediaSource(sourceOfSong: String): ProgressiveMediaSource {

        val dataSourceFactory = DefaultDataSourceFactory(
            this,
            getUserAgent(this, "ApplicationName")
        )

        return ProgressiveMediaSource.Factory(dataSourceFactory)
            .createMediaSource(
                Uri.parse(sourceOfSong)
            )
    }

I wanted to play a random song from the playlist so when I used seekTo(windowIndex, playbackPosition) then the app crashed. Then, on doing Log.d of player.nextWindowIndex and player.previousWindowIndex I got -1 then I checked timeline through player.currentTimeline.isEmpty then I got true. But concatenatingMediaSource.size is giving the correct number of songs that I have added. I don't understand why is it happening?

from Empty Timeline despite adding ConcatenatingMediaSouce in ExoPlayer