Fixed possible NPEs if Keyframes in List being null

src/main/java/eu/crushedpixel/replaymod/holders/KeyframeComparator.java

@@ -6,6 +6,7 @@ public class KeyframeComparator implements Comparator<Keyframe> {
 
     @Override
     public int compare(Keyframe o1, Keyframe o2) {
+        if(o1 == null || o2 == null) return 0;
         return ((Integer) o1.getRealTimestamp()).compareTo(o2.getRealTimestamp());
     }