Error: "The name 'UI' does not exist in the current context"

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Drawing;
using GTA;
using GTA.Native;
using GTA.Math;
using NativeUI;

namespace ModMenu
{
    public class Class1 : Script
    {
        MenuPool modMenuPool;
        UIMenu mainMenu;

        UIMenuItem resetWantedLevel;
        public Class1()
        {
            Setup();

            Tick += OnTick;
            KeyDown += OnKeyDown;
        }
        void Setup()
        {
            modMenuPool = new MenuPool();
            mainMenu = new UIMenu("Mod Menu", "SELECT AN OPTION");
            modMenuPool.Add(mainMenu);

            resetWantedLevel = new UIMenuItem("Reset Wanted Level");
            mainMenu.AddItem(resetWantedLevel);

            mainMenu.OnItemSelect += OnMainMenuItemSelect;
        }

        void OnMainMenuItemSelect(UIMenu sender, UIMenuItem item, int index)
        {
            if (item == resetWantedLevel)
            {
                if (Game.Player.WantedLevel == 0)
                {
                    UI.ShowSubtitle("You are not wanted");
                }
                else
                {
                    Game.Player.WantedLevel = 0;
                }
            }


        }
        void OnTick(object sender, EventArgs e)
        {

            if (modMenuPool != null)
                modMenuPool.ProcessMenus();
        }

        void OnKeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.H && !modMenuPool.IsAnyMenuOpen())
            {
                mainMenu.Visible = !mainMenu.Visible;
            }
        }

    }
}