fix(dialogs): keep a failing cancel in the title-bar close handler from ending the app

OnClosing is async void, so an exception from the view model's cancel
path was unobserved and crashed the process. It is now logged and the
dialog stays open.

YKanBan/Views/Dialogs/DialogWindow.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using Avalonia.Controls;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
@@ -85,6 +86,12 @@ public class DialogWindow : Window
         {
             await _viewModel.RequestCancelAsync();
         }
+        catch (Exception exception)
+        {
+            // An async void handler must not let a failure escape and end the
+            // process; the dialog simply stays open.
+            Debug.WriteLine(exception);
+        }
         finally
         {
             _cancelPending = false;