Different template, but essentially the same problem (worked in 7.4 died in 8.x)
Its an old unsupported RocketTheme gantry 4 joomla template in my case, but the following workaround is a fix for PHP8... maybe it will lead you to something similar?
`In /var/www/[redacted]/html/plugins/system/gantry/overrides/3.3/2.5/com_content/category/
I made the following changes to default_articles.php :
Line 17:
Old:
$params = &$this->item->params;
New:
$params = null;
if (property_exists($this,"item")){ $params = &$this->item->params; }
** IT'S A FIX !!`