Wrapping AdMediaSource causes a runtime crash

[arazabishov]

Issue description

Placing instance of AdMediaSource within a composite media source like ConcatenatingMediaSource will cause a crash within AdsMediaSource.prepareSourceInternal in the Assertions.checkArgument(isTopLevelSource). As far as I understand, the AdMediaSource is not supposed to be wrapped into another MediaSource, which seems counterintuitive to me. Documentation also doesn't describe this limitation in a clear way.

Reproduction steps

It is easily reproducible in the imademo app. Wrap AdMediaSource into ConcatenatingMediaSource before sending it into ExoPlayer.

Version of ExoPlayer being used

2.8.2

Device(s) and version(s) of Android being used

Android 8.1.0, Google Pixel XL

A full bug report captured from the device

2018-07-27 13:53:13.112 3697-3831/com.google.android.exoplayer2.imademo E/ExoPlayerImplInternal: Internal runtime error.
    java.lang.IllegalArgumentException
        at com.google.android.exoplayer2.util.Assertions.checkArgument(Assertions.java:39)
        at com.google.android.exoplayer2.source.ads.AdsMediaSource.prepareSourceInternal(AdsMediaSource.java:312)
        at com.google.android.exoplayer2.source.BaseMediaSource.prepareSource(BaseMediaSource.java:137)
        at com.google.android.exoplayer2.source.CompositeMediaSource.prepareChildSource(CompositeMediaSource.java:109)
        at com.google.android.exoplayer2.source.ConcatenatingMediaSource.addMediaSourceInternal(ConcatenatingMediaSource.java:554)
        at com.google.android.exoplayer2.source.ConcatenatingMediaSource.addMediaSourcesInternal(ConcatenatingMediaSource.java:560)
        at com.google.android.exoplayer2.source.ConcatenatingMediaSource.prepareSourceInternal(ConcatenatingMediaSource.java:390)
        at com.google.android.exoplayer2.source.BaseMediaSource.prepareSource(BaseMediaSource.java:137)
        at com.google.android.exoplayer2.ExoPlayerImplInternal.prepareInternal(ExoPlayerImplInternal.java:396)
        at com.google.android.exoplayer2.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:286)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:164)
        at android.os.HandlerThread.run(HandlerThread.java:65)
    
    
    --------- beginning of crash
2018-07-27 13:53:13.113 3697-3831/com.google.android.exoplayer2.imademo E/AndroidRuntime: FATAL EXCEPTION: ExoPlayerImplInternal:Handler
    Process: com.google.android.exoplayer2.imademo, PID: 3697
    java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.exoplayer2.source.ads.AdsMediaSource$ComponentListener.release()' on a null object reference
        at com.google.android.exoplayer2.source.ads.AdsMediaSource.releaseSourceInternal(AdsMediaSource.java:382)
        at com.google.android.exoplayer2.source.BaseMediaSource.releaseSource(BaseMediaSource.java:150)
        at com.google.android.exoplayer2.source.CompositeMediaSource.releaseSourceInternal(CompositeMediaSource.java:65)
        at com.google.android.exoplayer2.source.ConcatenatingMediaSource.releaseSourceInternal(ConcatenatingMediaSource.java:423)
        at com.google.android.exoplayer2.source.BaseMediaSource.releaseSource(BaseMediaSource.java:150)
        at com.google.android.exoplayer2.ExoPlayerImplInternal.resetInternal(ExoPlayerImplInternal.java:802)
        at com.google.android.exoplayer2.ExoPlayerImplInternal.stopInternal(ExoPlayerImplInternal.java:731)
        at com.google.android.exoplayer2.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:353)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:164)
        at android.os.HandlerThread.run(HandlerThread.java:65)

[andrewlewis]

#3750 tracks removing this limitation. I'll add a note to the class javadoc to document it.