Resolve an issue where changes to the main window's positioning could cause the application to crash if a modal was dismissed beforehand. (#6223)

src/Ryujinx.Ava/UI/Helpers/ContentDialogHelper.cs

@@ -336,6 +336,11 @@ namespace Ryujinx.Ava.UI.Helpers
 
                 void OverlayOnPositionChanged(object sender, PixelPointEventArgs e)
                 {
+                    if (_contentDialogOverlayWindow is null)
+                    {
+                        return;
+                    }
+
                     _contentDialogOverlayWindow.Position = parent.PointToScreen(new Point());
                 }