Today Wen I Cloned CustomSidebar Extension repo from Gerrit. I saw that It has no file in Its repo.
https://phabricator.wikimedia.org/diffusion/ECSB/repository/master/
And There is no mention of that Extension has archived.
https://www.mediawiki.org/wiki/Extension:CustomSidebar
The link https://www.mediawiki.org/wiki/Extension:CustomSidebar does not say anywhere that https://phabricator.wikimedia.org/diffusion/ECSB/repository/master/ has anything to do with https://www.mediawiki.org/wiki/Extension:CustomSidebar ...
In any case, might consider archiving on that page on mediawiki.org and removing an empty repository in Gerrit.
Looks like the maintainer of the extension described on https://www.mediawiki.org/wiki/Extension:CustomSidebar has no Phabricator account and has not been active lately: https://tools.wmflabs.org/guc/?user=Frantik
It seems that He hosted Extension:CustomSidebar on http://swiftlytilting.com/downloads/CustomSidebar.0.3.0.zip