Unable to play some videos.

## 🐛 Bug Report

unable to play some videos, as it throw an exception
### Expected behavior
The video should play normaly, as its type is an `mp4`
### Reproduction steps
The video is of type `mp4`, just by playing it, it throw some kind of java exception.

#### Exception detalj
```
Com.Google.Android.Exoplayer2.ExoPlaybackException: Exception of type 'Com.Google.Android.Exoplayer2.ExoPlaybackException' was thrown. ---> Java.Lang.NullPointerException: Attempt to invoke interface method 'com.google.android.exoplayer2.PlayerMessage com.google.android.exoplayer2.ExoPlayer.createMessage(com.google.android.exoplayer2.PlayerMessage$Target)' on a null object reference
--- End of inner exception stack trace ---
--- End of managed Com.Google.Android.Exoplayer2.ExoPlaybackException stack trace ---
com.google.android.exoplayer2.ExoPlaybackException
at com.google.android.exoplayer2.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:354)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:214)
at android.os.HandlerThread.run(HandlerThread.java:65)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'com.google.android.exoplayer2.PlayerMessage com.google.android.exoplayer2.ExoPlayer.createMessage(com.google.android.exoplayer2.PlayerMessage$Target)' on a null object reference
at com.google.android.exoplayer2.source.ConcatenatingMediaSource.scheduleListenerNotification(ConcatenatingMediaSource.java:513)
at com.google.android.exoplayer2.source.ConcatenatingMediaSource.handleMessage(ConcatenatingMediaSource.java:495)
at com.google.android.exoplayer2.ExoPlayerImplInternal.deliverMessage(ExoPlayerImplInternal.java:861)
at com.google.android.exoplayer2.ExoPlayerImplInternal.sendMessageToTarget(ExoPlayerImplInternal.java:829)
at com.google.android.exoplayer2.ExoPlayerImplInternal.sendMessageInternal(ExoPlayerImplInternal.java:811)
at com.google.android.exoplayer2.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:328)
... 3 more

```

I could post the video if you like.
### Configuration

**Version:** 0.6.8

**Platform:**
- [ ] :robot: Android
- [ ] :monkey: Xamarin.Forms

Start by reproducing playback of the reported MP4 on XamarinMediaManager 0.6.8 and inspect the ExoPlayer stack trace, especially ConcatenatingMediaSource and the null player reference. Done means the affected videos play without the reported ExoPlaybackException, with behavior checked on the applicable platform.

Written by the indexing model from the issue text.