Reminds me of T419063: PHP Warning: Undefined property: stdClass::$cat_title

There is a comment in Category about this situation

		# NOTE: the row often results from a LEFT JOIN on categorylinks. This may result in
		#       all the cat_xxx fields being null, if the category page exists, but nothing
		#       was ever added to the category. This case should be treated link an empty
		#       category, if possible.

		if ( $row->cat_title === null ) {

But with the rewrite for virutal domain from 23a9dfc624dfc9887d6650289ca5a1234e7c5868 there is no left join which would provide the null columns. In this special case the cat_* columns are not there at all.

Not sure how to trigger this special case. Maybe the row in category is from a defered update or job. When looking at the linked url the category page and the subcat were created in the same minute.

Change #1268289 had a related patch set uploaded (by Zabe; author: Zabe):

[mediawiki/core@master] CategoryViewer: Always set category fields in the query

https://gerrit.wikimedia.org/r/1268289

Change #1268289 merged by jenkins-bot:

[mediawiki/core@master] CategoryViewer: Always set category fields in the query

https://gerrit.wikimedia.org/r/1268289