Since MediaWiki 1.25, the suggested method of registering extensions has been to use
wfLoadExtension() instead of a direct
require_once. AddBodyClass currently (v1.2.0 from 2016-03-21) still uses the old style, so I’m opening this issue to suggest converting the extension to use the new method.
I’m also preparing a pull request with the minimal changes that I think should accomplish this, and I’ll post it next.