I see this error in a few support threads but not with my situation.
While updating an article, I opened another tab to look at the front end to see how the changes look. When I did so, the front end displayed the error. I can still enter the back end and make changes, only the front end is not showing.
Using FTP I can see the only files changed are akeeba admin tools files. Specifically cli/admintools-dbrepair.php and cli/admintools-filescanner.php. The temp folder and logs folder also showed updates, but they are empty folders.
From the backend I updated the akeeba admin pro plugin but it did not fix the error on the front end.
I'm not really sure if it's an Akeeba issue since nothing was updated on the backend, but those are the only files changed. When I run an error log I get this:
Call stack # Function Location 1 () JROOT/templates/risen_hope/vertex/responsive/responsive_mobile_menu/sidebar_url.php:89 2 require() JROOT/templates/risen_hope/vertex/responsive/responsive_mobile_menu/sidebar.php:121 3 require() JROOT/templates/risen_hope/vertex/responsive/responsive_mobile_menu.php:324 4 include() JROOT/templates/risen_hope/vertex/responsive/responsive_mobile_sidebar.php:70 5 require() JROOT/templates/risen_hope/vertex/includes/vertex_includes_top.php:4 6 require_once() JROOT/templates/risen_hope/index.php:174 7 require() JROOT/libraries/src/Document/HtmlDocument.php:668 8 Joomla\CMS\Document\HtmlDocument->_loadTemplate() JROOT/libraries/src/Document/HtmlDocument.php:730 9 Joomla\CMS\Document\HtmlDocument->_fetchTemplate() JROOT/libraries/src/Document/HtmlDocument.php:545 10 Joomla\CMS\Document\HtmlDocument->parse() JROOT/libraries/src/Application/CMSApplication.php:1030 11 Joomla\CMS\Application\CMSApplication->render() JROOT/libraries/src/Application/SiteApplication.php:778 12 Joomla\CMS\Application\SiteApplication->render() JROOT/libraries/src/Application/CMSApplication.php:202 13 Joomla\CMS\Application\CMSApplication->execute()JROOT/index.php:49