Download media playlist would crash

[RicardoJiang]

Issue description

when download hls media playlist videos in demo it will crash
FATAL EXCEPTION: main
Process: com.google.android.exoplayer2.demo, PID: 11416
java.lang.NullPointerException
at com.google.android.exoplayer2.util.Assertions.checkNotNull(Assertions.java:110)
at com.google.android.exoplayer2.source.hls.offline.HlsDownloadHelper.getDownloadAction(HlsDownloadHelper.java:100)
at com.google.android.exoplayer2.source.hls.offline.HlsDownloadHelper.getDownloadAction(HlsDownloadHelper.java:41)
at com.google.android.exoplayer2.demo.DownloadTracker$StartDownloadDialogHelper.onClick(DownloadTracker.java:298)
at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:163)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:150)
at android.app.ActivityThread.main(ActivityThread.java:5546)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:794)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:684)

Reproduction steps

just download media playlist video in demo
for example
"name": "Apple TS media playlist",
"uri": "https://devstreaming-cdn.apple.com/videos/streaming/examples/bipbop_4x3/gear1/prog_index.m3u8"

Version of ExoPlayer being used

latest

[tonihei]

Duplicate of #4396. And already fixed.