Fixed a bug where dekstop-directory isn't set

The program should make desktop-directory the proper ~/Desktop or ~/ directory if the value is 'null'. There was a bug where that just did not happen. Fixed that in this patch.

desktopIconsUtil.js

@@ -29,6 +29,7 @@ function getDesktopDir() {
     let desktopPath = Prefs.desktopSettings.get_string('desktop-directory');
     if(desktopPath == 'null'){
     	desktopPath = GLib.get_user_special_dir(GLib.UserDirectory.DIRECTORY_DESKTOP);
+    	Prefs.desktopSettings.set_string('desktop-directory', desktopPath);
     }
     return Gio.File.new_for_commandline_arg(desktopPath);
 }