Thread:Extension talk:CategoryTree/Problem loading data. Please wait a moment and try again

With MW git master + CategoryTree git master, my category tree shows one level. Clicking on the arrow produces the message: Problem loading data. Please wait a moment and try again

If I turn on debugging, I see the following in the backtrace:

[GlobalTitleFail] MessageCache::parse called by CategoryTreePage::execute/Message::parse/Message::toString/Message::parseText/MessageCache::parse with no title set.

Could be related to this: https://phabricator.wikimedia.org/T25307