Found in Logstash:
PHP Error: Argument 1 passed to WikibaseQuality\ConstraintReport\Api\ResultsCache::delete() must be an instance of Wikibase\DataModel\Entity\EntityId, null given
#0 /srv/mediawiki/php-1.31.0-wmf.27/extensions/WikibaseQualityConstraints/src/Api/ResultsCache.php(85): MWExceptionHandler::handleError(integer, string, string, integer, array, array)
#1 /srv/mediawiki/php-1.31.0-wmf.27/extensions/WikibaseQualityConstraints/src/WikibaseQualityConstraintsHooks.php(80): WikibaseQuality\ConstraintReport\Api\ResultsCache->delete(NULL)
#2 /srv/mediawiki/php-1.31.0-wmf.27/includes/Hooks.php(177): WikibaseQuality\ConstraintReport\WikibaseQualityConstraintsHooks::onArticlePurge(WikiPage)
#3 /srv/mediawiki/php-1.31.0-wmf.27/includes/Hooks.php(205): Hooks::callHook(string, array, array, NULL)
#4 /srv/mediawiki/php-1.31.0-wmf.27/includes/page/WikiPage.php(1171): Hooks::run(string, array)
#5 /srv/mediawiki/php-1.31.0-wmf.27/includes/page/Article.php(2133): WikiPage->doPurge()
#6 /srv/mediawiki/php-1.31.0-wmf.27/includes/actions/PurgeAction.php(45): Article->doPurge()
#7 /srv/mediawiki/php-1.31.0-wmf.27/includes/actions/PurgeAction.php(66): PurgeAction->onSubmit(array)
#8 /srv/mediawiki/php-1.31.0-wmf.27/includes/MediaWiki.php(500): PurgeAction->show()
#9 /srv/mediawiki/php-1.31.0-wmf.27/includes/MediaWiki.php(294): MediaWiki->performAction(Article, Title)
#10 /srv/mediawiki/php-1.31.0-wmf.27/includes/MediaWiki.php(861): MediaWiki->performRequest()
#11 /srv/mediawiki/php-1.31.0-wmf.27/includes/MediaWiki.php(524): MediaWiki->main()
#12 /srv/mediawiki/php-1.31.0-wmf.27/index.php(42): MediaWiki->run()
#13 /srv/mediawiki/w/index.php(3): include(string)
#14 {main}
Only happened once in the past 24 hours. Request URL indicates someone was purging an item (let’s redact which one it was, it probably doesn’t matter – but the item exists and has no recent log actions that I can see), which might explain why it’s not a frequent error.