Fix shops resetting itemstack size to qty 1

src/main/java/com/alttd/playershops/shop/PlayerShop.java

@@ -154,7 +154,7 @@ public class PlayerShop {
     }
 
     public boolean isInitialized() {
-        return type != ShopType.NONE && itemStack != null;
+        return type != ShopType.NONE;
     }
 
     public void updateSign() {
@@ -368,8 +368,12 @@ public class PlayerShop {
         if (Util.callCancellableEvent(shopItemChangeEvent))
             return; // cancelled by another plugin, does this need logging?
 
-        this.itemStack = itemStack;
-        this.itemStack.setAmount(this.amount != 0 ? this.amount : 1);
+        if (itemStack == null) {
+            this.itemStack = null;
+        } else {
+            this.itemStack = itemStack.clone();
+            this.itemStack.setAmount(this.amount != 0 ? this.amount : 1);
+        }
         setDirty(true);
         update();
     }