TOPIC: Final file is impossible to download

Dear ShopFile admin,
I bought some months ago the plugin quickselljoomla3_pro but now that I've installed it I have this big problem: after the paypal transition (sandbox or real) the final file (.pdf 5 or zip, crypted or not) is it impossibile to download. I get every time the message: "Fatal error: Call to undefined method Joomla\CMS\Document\RawDocument::addFavicon() in /web/htdocs/www.xxx.it/home/Beta/templates/shaper_strings/error.php on line 27"
I'm using Joomla 3.8.0 with Sharper Strings template and Helix 3 Framwork.
Can you help me?
Thank you very much,

Francesco