Fix window lifecycle management to prevent exceptions when closing main window with child windows

Co-authored-by: niels9001 <9866362+niels9001@users.noreply.github.com>

Author: Copilot

WinUIGallery/App.xaml.cs

@@ -8,6 +8,7 @@
 using Microsoft.Windows.AppNotifications.Builder;
 using Microsoft.Windows.BadgeNotifications;
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 using Windows.ApplicationModel.Activation;
@@ -60,6 +61,23 @@ protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs ar
         MainWindow.Closed += (s, e) =>
         {
             BadgeNotificationManager.Current.ClearBadge();
+            
+            // Close all remaining active windows to prevent resource disposal conflicts
+            var activeWindows = new List<Window>(WindowHelper.ActiveWindows);
+            foreach (var window in activeWindows)
+            {
+                if (window != s) // Don't try to close the window that's already closing
+                {
+                    try
+                    {
+                        window.Close();
+                    }
+                    catch
+                    {
+                        // Ignore any exceptions during cleanup
+                    }
+                }
+            }
         };
     }
 

WinUIGallery/Samples/ControlPages/CreateMultipleWindowsPage.xaml.cs

@@ -3,6 +3,7 @@
 
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
+using WinUIGallery.Helpers;
 using WinUIGallery.Pages;
 
 namespace WinUIGallery.ControlPages;
@@ -17,6 +18,7 @@ public CreateMultipleWindowsPage()
     private void createNewWindow_Click(object sender, RoutedEventArgs e)
     {
         var newWindow = new MainWindow();
+        WindowHelper.TrackWindow(newWindow);
         newWindow.Activate();
 
         var targetPageType = typeof(HomePage);