Fix exception raised if something is added to the queue while Chords isn't present.

src/main/java/moe/nekojimi/chords/QueueManager.java

@@ -51,7 +51,7 @@ public class QueueManager extends QueueThing<Track, Track>
 //        if (inputQueue.size() < QUEUE_TARGET_SIZE)
 //            demandInput(QUEUE_TARGET_SIZE - inputQueue.size());
 
-        if (!handler.isPlaying() || handler.getCurrentTrack() == null)
+        if (handler != null && !handler.isPlaying() || handler.getCurrentTrack() == null)
             handler.requestTrack();
     }