Fix some NPE caused by null itemstacks

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

@@ -313,7 +313,7 @@ public class PlayerShop {
     }
 
     private TransactionError executeGambleTransaction(Player player) {
-        ItemStack itemStack = getRandomItem();
+        ItemStack itemStack = getItemStack();
         if (itemStack == null)
             return TransactionError.INSUFFICIENT_FUNDS_SHOP;
 
@@ -419,9 +419,11 @@ public class PlayerShop {
         transactions.add(transaction);
     }
 
-    private ItemStack getRandomItem() {
+    public ItemStack getItemStack() {
         if (!isInitialized())
             return null;
+        if (this.getType() != ShopType.GAMBLE)
+            return itemStack;
         if (this.getInventory().isEmpty())
             return null;
         ArrayList<ItemStack> contents = new ArrayList<>();