Proper instance NPC deletion.

L2J_Mobius_Underground/java/com/l2jmobius/gameserver/model/events/timers/TimerHolder.java

@@ -112,7 +112,7 @@ public class TimerHolder<T> implements Runnable
 			return false;
 		}
 		
-		_task.cancel(false);
+		_task.cancel(true);
 		_cancelScript.onTimerCancel(this);
 		return true;
 	}

L2J_Mobius_Underground/java/com/l2jmobius/gameserver/model/instancezone/Instance.java

@@ -642,6 +642,7 @@ public final class Instance implements IIdentifiable, INamable
 	public void removeNpcs()
 	{
 		_spawns.forEach(SpawnTemplate::despawnAll);
+		_npcs.forEach(L2Npc::deleteMe);
 		_npcs.clear();
 	}