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

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

@@ -313,11 +313,13 @@ public final class CombatUtils {
                         target.setMetadata(mcMMO.customVisibleKey, new FixedMetadataValue(mcMMO.p, oldNameVisible));
                     }
 
+                    if (target.isValid()) {
                         new MobHealthDisplayUpdaterTask(target).runTaskLater(mcMMO.p, displayTime * 20); // Clear health display after 3 seconds
                     }
                 }
             }
         }
+    }
 
     /**
      * Attempt to damage target for value dmg with reason CUSTOM