Memory Leak when creating Document Windows

namespace TestDocumentWindowMemoryLeak
{
    public partial class Form1 : Form
    {
        private string _WindowKey = string.Empty;
        public Form1()
        {
            InitializeComponent();
        }

        /// <summary>
        /// timer1 is triggered for every 3 seconds, in which the existing document window 
        /// is closed and a new one is created.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void timer1_Tick(object sender, EventArgs e)
        {
            if (_WindowKey != string.Empty)
            {
                this.dockManager1.DocumentWindows[_WindowKey].Close();
            }

            _WindowKey = Guid.NewGuid().ToString();

            ActiproSoftware.UIStudio.Dock.DocumentWindow theWindow = new ActiproSoftware.UIStudio.Dock.DocumentWindow(this.dockManager1, _WindowKey, _WindowKey, null, new UserControl1());
            theWindow.Activate(false);
        }
    }
}