Notice Undefined index: customclass

Hello, this Notice exits because I wrong something?

Notice: Undefined index: customclass in /var/www/ilsorriso/wp-content/plugins/standout-css3-buttons/standout-css3-buttons.php on line 479

https://ilsorrisodeimieibimbi.org/turismo-responsabile/

Thanks

https://www.ads-software.com/plugins/standout-css3-buttons/