Set ShutDown policy

src/PicView/Program.cs

@@ -17,7 +17,7 @@ namespace PicView
         // yet and stuff might break.
         [STAThread]
         public static void Main(string[] args) => BuildAvaloniaApp()
-            .StartWithClassicDesktopLifetime(args);
+            .StartWithClassicDesktopLifetime(args, Avalonia.Controls.ShutdownMode.OnMainWindowClose);
 
         // Avalonia configuration, don't remove; also used by visual designer.
         public static AppBuilder BuildAvaloniaApp()