Fix NPE in shopByMaterial

src/main/java/com/alttd/playershops/handler/ShopHandler.java

@@ -217,6 +217,9 @@ public class ShopHandler {
             if (!shop.isInitialized()) {
                 continue;
             }
+            if (shop.getItemStack() == null) {
+                continue;
+            }
 
             Material material = shop.getShopItemType();
             List<PlayerShop> shopList = shopByMaterialTreeSet.computeIfAbsent(material, v -> new ArrayList<>());