Exception when clicking void slot

[Zoriot]

You get an exception when you click on a slot where no item is set

[21:19:00 ERROR]: Could not pass event InventoryClickEvent to BlockPalletGUI v1.0
java.lang.NullPointerException: Cannot invoke "org.bukkit.inventory.ItemStack.getType()" because "currentItem" is null
        at org.example.bte.blockPalletGUI.InventoryClickHandler.onInventoryClick(InventoryClickHandler.java:29) ~[BlockPalletGUI-0.1.jar:?]
        at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor10.execute(Unknown Source) ~[?:?]

Please fix that, should be possible with an early return