Error
MediaWiki version: 1.36.0-wmf.2
Call to a member function purgeCache() on null
Impact
There's been about 370 of these recently (in the past hour) on 1.36.0-wmf.2, but don't seem to have happened earlier.
Seems like a clear programming error, but might mask other problems.
I don't think this warrants a rollback of train at this rate, but it should be fixed for next train (which starts tomorrow).