SelectionChanged Stops Firing After Last Window Closed

<Window
    x:Class="RibbonSample.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
    Title="Window1">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <Button Content="Add Window" Click="Button_Click" />
        <Grid Grid.Row="1">
            <docking:DockSite x:Name="uxDockSite_DockSite">
                <docking:Workspace>
                    <docking:TabbedMdiHost>
                        <docking:TabbedMdiContainer SelectionChanged="uxTabbedMdiContainer_TabContainer_SelectionChanged">                       
                            <docking:DocumentWindow Title="One" Content="One"/>
                            <docking:DockingWindow Title="Two" Content="Two" />
                            <docking:DockingWindow Title="Three" Content="Three"/>
                            <docking:DockingWindow Title="Four" Content="Four"/>
                        </docking:TabbedMdiContainer>
                    </docking:TabbedMdiHost>
                </docking:Workspace>
            </docking:DockSite>
        </Grid>
    </Gri

using System;
using System.Windows;
using System.Windows.Controls;
using ActiproSoftware.Windows.Controls.Docking;

namespace RibbonSample
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1
    {
        public Window1()
        {
            InitializeComponent();
        }

        private void uxTabbedMdiContainer_TabContainer_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            Console.WriteLine("Selection Changed");
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            DocumentWindow window = new DocumentWindow(uxDockSite_DockSite);
            window.Title = "New Window";
            window.Activate();
        }
    }
}