Fixed in the next release. This is an E_NOTICE error. It is simply sent to the log unless debug mode is turned on (debug mode treats all E_ as E_ERROR e.g. Fatal Error).
This error prevents the titles of Special pages from being added to XenForo's search index (they still appear in VaultWiki's index). The upgrade to 4.0.6 will attempt to re-add Special pages to the XenForo index.