exception 'MWException' with message 'Language::isValidBuiltInCode must be passed a string, NULL given' in /srv/web/XXX/docroot/languages/Language.php:256
Stack trace:
#0 /srv/web/XXX/docroot/languages/Language.php(188): Language::isValidBuiltInCode(NULL)
#1 /srv/web/XXX/docroot/languages/Language.php(169): Language::newFromCode(NULL)
#2 /srv/web/XXX/docroot/includes/resourceloader/ResourceLoaderContext.php(164): Language::factory(NULL)
#3 /srv/web/XXX/docroot/includes/resourceloader/ResourceLoaderModule.php(117): ResourceLoaderContext->getDirection()
#4 /srv/web/XXX/docroot/includes/resourceloader/ResourceLoaderUserCSSPrefsModule.php(87): ResourceLoaderModule->getFlip(Object(ResourceLoaderContext))
#5 /srv/web/XXX/docroot/includes/resourceloader/ResourceLoader.php(721): ResourceLoaderUserCSSPrefsModule->getStyles(Object(ResourceLoaderContext))
#6 /srv/web/XXX/docroot/includes/OutputPage.php(2631): ResourceLoader->makeModuleResponse(Object(ResourceLoaderContext), Array)
#7 /srv/web/XXX/docroot/includes/OutputPage.php(3405): OutputPage->makeResourceLoaderLink(Array, 'styles')
#8 /srv/web/XXX/docroot/includes/OutputPage.php(2428): OutputPage->buildCssLinks()
#9 /srv/web/XXX/docroot/includes/SkinTemplate.php(461): OutputPage->headElement(Object(SkinMrLocale))
#10 /srv/web/XXX/docroot/includes/OutputPage.php(2003): SkinTemplate->outputPage()
#11 /srv/web/XXX/docroot/includes/Wiki.php(405): OutputPage->output()
#12 /srv/web/XXX/docroot/includes/Wiki.php(593): MediaWiki->finalCleanup()
#13 /srv/web/XXX/docroot/includes/Wiki.php(502): MediaWiki->main()
#14 /srv/web/XXX/docroot/index.php(58): MediaWiki->run()
#15 {main}
Version: 1.20.x
Severity: blocker