DockSite.WindowsClosing is not called when inside a UserControl

I have a docksite which when attached to my main window the WindowsClosing is called after calling Application.Current.Shutdown();, however, I just moved the docksite to a UserControl that is loaded once my user logs into my application.  Now when I call Application.Current.Shutdown(); the WindowsClosing event is not triggered.

    public partial class MainControl : UserControl
    {
        public MainControl()
        {
            InitializeComponent();

            dockSite.WindowsClosing += DockSite_WindowsClosing;
        }
    }