prevent player throw out potion or some else widget while click the shop sign

Author: Ghost-chu

src/main/java/org/maxgamer/quickshop/shop/ContainerShop.java

@@ -937,7 +937,7 @@ public void refresh() {
                 attachedShop.refresh();
             }
             // Don't make an item for this chest if it's a left shop.
-            if (!isLeftShop && !isDisableDisplay()) {
+            if (!isLeftShop && !isDisableDisplay() && displayItem != null) {
                 displayItem.spawn();
             }
         }