Spigot issue test code plugin

Main.java

package com.github.stefvanschiedev.invframework.test;

import org.bukkit.Bukkit;
import org.bukkit.Material;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.HumanEntity;
import org.bukkit.inventory.Inventory;
import org.bukkit.inventory.ItemStack;
import org.bukkit.plugin.java.JavaPlugin;

public class Main extends JavaPlugin {

    ItemStack itemStack = new ItemStack(Material.GREEN_WOOL);
    Inventory inventory = Bukkit.createInventory(null, 54, "Test");

    @Override
    public void onEnable() {
        inventory.addItem(itemStack);
    }

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        if (!(sender instanceof HumanEntity))
            return false;

        if (args[0].equalsIgnoreCase("open")) {
            ((HumanEntity) sender).openInventory(inventory);
        } else if (args[0].equalsIgnoreCase("change")) {
            itemStack.setType(Material.RED_WOOL);
        }

        return true;
    }
}

plugin.yml

main: com.github.stefvanschiedev.invframework.test.Main
name: IFTest
version: 0.0.0
api-version: 1.13

commands:
  gui: