Call to undefined method JAdminCssMenu::addChild in backend
From Joomla! Documentation
In Joomla! 3.8, the administrator menu module was updated to include new features. As a result of the changes, some extensions may have been broken by the 3.8 update.
Errors reported[edit]
Call to undefined method JAdminCssMenu::addChild
Versions affected[edit]
General Information
This pertains only to Joomla! version(s): 3.8.0
What is the cause[edit]
Changes in the administrator menu module to support new features.
How to fix[edit]
Ensure all extensions are updated prior to updating Joomla.