J3.x: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.
Call to undefined method JAdminCssMenu::addChild
This pertains only to Joomla! version(s): 3.8.0
What is the cause
Changes in the administrator menu module to support new features.
How to fix
Ensure all extensions are updated prior to updating Joomla.