Fix #12485 when main window is not visible (#16303)

src/Avalonia.Native/AvaloniaNativeMenuExporter.cs

@@ -76,10 +76,14 @@ namespace Avalonia.Native
                 var dialog = new AboutAvaloniaDialog();
 
                 if (Application.Current is
-                    { ApplicationLifetime: IClassicDesktopStyleApplicationLifetime { MainWindow: { } mainWindow } })
+                    { ApplicationLifetime: IClassicDesktopStyleApplicationLifetime { MainWindow: { IsVisible: true } mainWindow } })
                 {
                     await dialog.ShowDialog(mainWindow);   
                 }
+                else
+                {
+                    dialog.Show();
+                }
             };
             
             result.Add(aboutItem);