Object refference error

[Squirrel993]

Alright so I cannot wrap my head around this. First time using Rage UI. 
The error reads 

 

Quote

Object reference not set to an instance of an object.
Exception has been thrown by the target of an invocation.
UNHANDLED EXCEPTION IN COMMAND test FOR RESOURCE GTARP/GTARP
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.

using System;
using GTANetworkAPI;

namespace GTARP.Commands
{
    class UserCommands : Script
    {
        RAGE.Ui.HtmlWindow CEF;

        public void createBrowser()
        {
            CEF = new RAGE.Ui.HtmlWindow("resource://index.html");
        }

        [Command("test")]
        public void testCommand(Player player)
        {
            createBrowser();
            this.CEF.Active = true;
        }
    }
}