Bug fix related to closing windows

MainWindow.xaml.cs

@@ -26,22 +26,34 @@ namespace IPClab1;
 public sealed partial class MainWindow : Window
 {
     private readonly DispatcherTimer timer = new() { Interval = TimeSpan.FromMilliseconds(100) };
+    private bool IsClosed;
 
     public MainWindowViewModel ViewModel;
     public MainWindow(MainWindowViewModel viewModel)
     {
         InitializeComponent();
+
+        IsClosed = false;
+        Closed += MainWindow_Closed;
+
         ViewModel = viewModel;
+
         ExtendsContentIntoTitleBar = true;
         SetTitleBar(AppTitleBar);
+
         timer.Tick += Timer_Tick;
         timer.Start();
     }
 
+    private void MainWindow_Closed(object sender, WindowEventArgs args) => IsClosed = true;
+
     private void Timer_Tick(object? sender, object e)
     {
-        AppTitleBar.Title = Title;
-        ViewModel.RefreshInfo();
+        if (!IsClosed)
+        {
+            AppTitleBar.Title = Title;
+            ViewModel.RefreshInfo();
+        }
     }
 
     private void GridView_SizeChanged(object sender, SizeChangedEventArgs e)

ViewModels/MainWindowViewModel.cs

@@ -48,7 +48,8 @@ public partial class MainWindowViewModel(WindowListService windowListService, Wi
 
     partial void OnSelectedWindowChanged(WindowInfo value)
     {
-        (Width, Height) = _windowInteractionService.GetWindowSize(value.Hwnd);
+        if (value != null) 
+            (Width, Height) = _windowInteractionService.GetWindowSize(value.Hwnd);
     }
 
     [ObservableProperty]