Fix immutable lists

src/main/java/emu/grasscutter/game/managers/InventoryManager.java

@@ -696,7 +696,7 @@ public class InventoryManager {
 		}
 		
 		// Pay materials and mora if possible
-		List<ItemParamData> costs = proudSkill.getCostItems();  // Can this be null?
+		List<ItemParamData> costs = new ArrayList<ItemParamData>(proudSkill.getCostItems());  // Can this be null?
 		if (proudSkill.getCoinCost() > 0) {
 			costs.add(new ItemParamData(202, proudSkill.getCoinCost()));
 		}

src/main/java/emu/grasscutter/server/packet/recv/HandlerBuyGoodsReq.java

@@ -56,7 +56,7 @@ public class HandlerBuyGoodsReq extends PacketHandler {
                 return;
             }
 
-            List<ItemParamData> costs = sg.getCostItemList();  // Can this even be null?
+            List<ItemParamData> costs = new ArrayList<ItemParamData>(sg.getCostItemList());  // Can this even be null?
             costs.add(new ItemParamData(202, sg.getScoin()));
             costs.add(new ItemParamData(201, sg.getHcoin()));
             costs.add(new ItemParamData(203, sg.getMcoin()));