Fixed NPE in TradeList, also there is no point to log it when it happens.

Contributed by Sahar.

L2J_Mobius_2.5_Underground/java/org/l2jmobius/gameserver/model/TradeList.java

@@ -182,9 +182,8 @@ public class TradeList
 		}
 		
 		final WorldObject o = World.getInstance().findObject(objectId);
-		if (!o.isItem())
+		if (!(o instanceof ItemInstance))
 		{
-			LOGGER.warning(_owner.getName() + ": Trying to add something other than an item!");
 			return null;
 		}