1.34.0.-wmf.8 introduced an undefined property PHP notice:
PHP Notice: Undefined property: stdClass::$value
#0 /srv/mediawiki/php-1.34.0-wmf.8/includes/api/ApiQueryQueryPage.php(125): MWExceptionHandler::handleError(integer, string, string, integer, array, array)
#1 /srv/mediawiki/php-1.34.0-wmf.8/includes/api/ApiQueryQueryPage.php(53): ApiQueryQueryPage->run()
#2 /srv/mediawiki/php-1.34.0-wmf.8/includes/api/ApiQuery.php(249): ApiQueryQueryPage->execute()
#3 /srv/mediawiki/php-1.34.0-wmf.8/includes/api/ApiMain.php(1595): ApiQuery->execute()
#4 /srv/mediawiki/php-1.34.0-wmf.8/includes/api/ApiMain.php(531): ApiMain->executeAction()
#5 /srv/mediawiki/php-1.34.0-wmf.8/includes/api/ApiMain.php(502): ApiMain->executeActionWithErrorHandling()
#6 /srv/mediawiki/php-1.34.0-wmf.8/api.php(87): ApiMain->execute()
#7 /srv/mediawiki/w/api.php(3): include(string)
#8 {main}It started happening on 2019-06-13 and happens every day around 03:00 UTC and lasts for an hour during which time approximately 2M such errors are logged. This suggests a periodic (maintenance?) script is inducing MW to do that.