#2947 - fix app crash in some error case with ExoPlayer.

(It does not address root problems in #2947 though).

core/src/main/java/de/danoeh/antennapod/core/service/playback/LocalPSMP.java

@@ -991,7 +991,7 @@ public class LocalPSMP extends PlaybackServiceMediaPlayer {
 
     private final MediaPlayer.OnErrorListener audioErrorListener =
             (mp, what, extra) -> {
-                if(mp.canFallback()) {
+                if(mp != null && mp.canFallback()) {
                     mp.fallback();
                     return true;
                 } else {