Don't handle removing the name if the entity is dead.

src/main/java/com/gmail/nossr50/util/skills/CombatUtils.java

@@ -313,7 +313,9 @@ public final class CombatUtils {
                         target.setMetadata(mcMMO.customVisibleKey, new FixedMetadataValue(mcMMO.p, oldNameVisible));
                     }
 
-                    new MobHealthDisplayUpdaterTask(target).runTaskLater(mcMMO.p, displayTime * 20); // Clear health display after 3 seconds
+                    if (target.isValid()) {
+                        new MobHealthDisplayUpdaterTask(target).runTaskLater(mcMMO.p, displayTime * 20); // Clear health display after 3 seconds
+                    }
                 }
             }
         }