this is somewhat frequent in the logs
/w/index.php?title=%E7%89%B9%E5%88%A5:MathShowImage&hash=773282d0f15f7cd9b50ac4b8268c896a&mode=mathml BadMethodCallException from line 29 of /srv/mediawiki/php-1.26wmf17/extensions/Graph/Graph.body.php: Call to a member function getIsPreview() on a non-object (NULL) {"exception":"[Exception BadMethodCallException] (/srv/mediawiki/php-1.26wmf17/extensions/Graph/Graph.body.php:29) Call to a member function getIsPreview() on a non-object (NULL)\n[stacktrace]\n#0 /srv/mediawiki/php-1.26wmf17/includes/Hooks.php(204): Graph\\Singleton::onParserAfterParse(Parser)\n#1 /srv/mediawiki/php-1.26wmf17/extensions/Math/MathTexvc.php(397): Hooks::run(string, array)\n#2 /srv/mediawiki/php-1.26wmf17/extensions/Math/SpecialMathShowImage.php(96): MathTexvc->getPng()\n#3 /srv/mediawiki/php-1.26wmf17/includes/specialpage/SpecialPage.php(384): SpecialMathShowImage->execute(NULL)\n#4 /srv/mediawiki/php-1.26wmf17/includes/specialpage/SpecialPageFactory.php(553): SpecialPage->run(NULL)\n#5 /srv/mediawiki/php-1.26wmf17/includes/MediaWiki.php(249): SpecialPageFactory::executePath(Title, RequestContext)\n#6 /srv/mediawiki/php-1.26wmf17/includes/MediaWiki.php(677): MediaWiki->performRequest()\n#7 /srv/mediawiki/php-1.26wmf17/includes/MediaWiki.php(474): MediaWiki->main()\n#8 /srv/mediawiki/php-1.26wmf17/index.php(41): MediaWiki->run()\n#9 /srv/mediawiki/w/index.php(3): include(string)\n#10 {main}\n"}