Fixed nullability issues

library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java

@@ -92,7 +92,7 @@ public class SimpleExoPlayer implements ExoPlayer, Player.VideoComponent, Player
   private AudioAttributes audioAttributes;
   private float audioVolume;
   private MediaSource mediaSource;
-  private List<Cue> currentCues;
+  private @Nullable List<Cue> currentCues;
 
   /**
    * @param renderersFactory A factory for creating {@link Renderer}s to be used by the instance.
@@ -503,7 +503,9 @@ public class SimpleExoPlayer implements ExoPlayer, Player.VideoComponent, Player
 
   @Override
   public void addTextOutput(TextOutput listener) {
-    listener.onCues(currentCues);
+    if(currentCues != null) {
+      listener.onCues(currentCues);
+    }
     textOutputs.add(listener);
   }