I'm trying to clean up remaining issues after the migration to J3.8 and in checking the server error_log file I found this (actually hundreds of these):
[18-Dec-2017 22:20:48 UTC] PHP Fatal error: Call to undefined method Joomla\CMS\Document\FeedDocument::addFavicon() in /home/ourSite/public_html/templates/jd_dallas/error.php on line 27

Which is:

//Favicon
if($favicon = $params->get('favicon')) {
    $doc->addFavicon( JURI::base(true) . '/' .  $favicon);
} else {
    $doc->addFavicon( $this->baseurl . '/templates/' . $this->template . '/images/favicon.ico' );
}

I'm not sure why this is causing a fatal error as the favicon.ico file is in the template images directory. I have commented this routine out and it didn't seem to cause any errors and the favicon is still in the browser tab.

Do you have any idea as to why this code is throwing a fatal error?

https://github.com/JoomShaper/Helix3/blob/master/template/error.php