MY ERROR///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
[16:15:49 ERROR]: Could not pass event InventoryClickEvent to spenchant v0.1
org.bukkit.event.EventException: null
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:500) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:485) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at net.minecraft.server.v1_12_R1.PlayerConnection.a(PlayerConnection.java:1893) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at net.minecraft.server.v1_12_R1.PacketPlayInWindowClick.a(SourceFile:33) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at net.minecraft.server.v1_12_R1.PacketPlayInWindowClick.a(SourceFile:10) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at net.minecraft.server.v1_12_R1.PlayerConnectionUtils$1.run(SourceFile:13) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_211]
        at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_211]
        at net.minecraft.server.v1_12_R1.SystemUtils.a(SourceFile:46) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:748) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:406) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577) [spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_211]
Caused by: java.lang.NullPointerException
        at spenchant.fadedlake64040.UpgradeGUI.onclick(UpgradeGUI.java:83) ~[?:?]
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_211]
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
        at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_211]
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:302) ~[spigot-1.12.2.jar:git-Spigot-79a30d7-acbc348]
        ... 15 more
MY CODE////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

public class UpgradeGUI implements CommandExecutor, Listener{
	public HashMap<String, Integer> ut = new HashMap<String, Integer>();
	public Inventory inv = Bukkit.createInventory(null, 54, "UpgradeGui");
	@Override
	public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) {
		if(cmd.getName().equalsIgnoreCase("upgrade"))
		{
			if(sender instanceof Player)
			{

				((Player) sender).openInventory(inv);

				ItemStack dhu = new ItemStack(Material.DIAMOND_HELMET);
				ItemMeta dhum = dhu.getItemMeta();
				dhum.setDisplayName(ChatColor.AQUA + "Unbreaking Upgrade");
				dhu.setItemMeta(dhum);
				inv.setItem(10, dhu);
			}

		}
		return false;
	}
	@EventHandler
	public void onPlayerJoin(PlayerJoinEvent e){
	    if(ut.containsKey(e.getPlayer().getName()) == false)
	    {
	        ut.put(e.getPlayer().getName(), 50);
	    }
	}
	@EventHandler
	public void onclick(InventoryClickEvent e) {
		if(e.getClickedInventory().getName().equals(inv.getName()))
		{
			if(e.getCurrentItem() == null || e.getCurrentItem().getType() == Material.AIR)
			{

				e.setCancelled(true);
				return;

			}
			else if(e.getCurrentItem().getType().equals(Material.DIAMOND_HELMET))
			{
				e.setCancelled(true);
				if(e.getCurrentItem().getItemMeta().getDisplayName().toLowerCase().contains("unbreaking"))
				{
					int i = ut.get(e.getWhoClicked().getUniqueId().toString());
					while(ut.containsKey(e.getWhoClicked().getUniqueId().toString()) == false)
					{
						e.getWhoClicked().sendMessage("no player id");
					}
					if(i >= 1)
					{
						if(e.getWhoClicked().getInventory().getHelmet().containsEnchantment(Enchantment.DURABILITY))
						{
							e.getWhoClicked().getInventory().getHelmet().addUnsafeEnchantment(Enchantment.DURABILITY, e.getWhoClicked().getInventory().getHelmet().getEnchantmentLevel(Enchantment.DURABILITY) + 1);
						}
					return;
					}
					else
					{
						e.getWhoClicked().sendMessage(ChatColor.DARK_RED + "You do not have enough upgrade tokens to do that");
					}
				}

			}
			else
			{
			e.setCancelled(true);
			}
		}
	}
}