MediaSession: Add a null check in MediaSessionImplApi21.setCallback

Bug: 24326977
Change-Id: Id07ae79010254c59647b6aac0641bfcb2f68582c
diff --git a/v4/java/android/support/v4/media/session/MediaSessionCompat.java b/v4/java/android/support/v4/media/session/MediaSessionCompat.java
index 62896f7..7b38a54 100644
--- a/v4/java/android/support/v4/media/session/MediaSessionCompat.java
+++ b/v4/java/android/support/v4/media/session/MediaSessionCompat.java
@@ -1966,7 +1966,8 @@
 
         @Override
         public void setCallback(Callback callback, Handler handler) {
-            MediaSessionCompatApi21.setCallback(mSessionObj, callback.mCallbackObj, handler);
+            MediaSessionCompatApi21.setCallback(mSessionObj,
+                    callback == null ? null : callback.mCallbackObj, handler);
         }
 
         @Override