Add a check if the shop block is an inventoryholder

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

@@ -148,9 +148,10 @@ public class PlayerShop {
     }
 
     public Inventory getInventory() {
-        // Could use a check if the block is still an InventoryHolder.
-        InventoryHolder container = (InventoryHolder) shopLocation.getBlock().getState();
-        return container.getInventory();
+        if (shopLocation.getBlock().getState() instanceof InventoryHolder inventoryHolder) {
+            return inventoryHolder.getInventory();
+        }
+        return null;
     }
 
     public boolean isInitialized() {