Edge case: Contao throws error when RSS feed contains form with antispam field in it

[Teetrinker]

What's needed to reproduce, afaik:
create news feeds, create rss feed, Exporteinstellungen: ganze Beiträge
create form with antispam form field, include form in news entry

Happened on an installation with Contao 4.13.49 + rocksolid/antispam 2.1.0

Stack Trace:

InvalidArgumentException:
The SessionBagInterface "contao_frontend" is not registered.

at vendor/symfony/http-foundation/Session/Storage/MockArraySessionStorage.php:189
at Symfony\Component\HttpFoundation\Session\Storage\MockArraySessionStorage->getBag('contao_frontend')
(vendor/symfony/http-foundation/Session/Session.php:261)
at Symfony\Component\HttpFoundation\Session\Session->getBag('contao_frontend')
(vendor/madeyourday/contao-rocksolid-antispam/src/Form/AntispamField.php:186)
at MadeYourDay\RockSolidAntispam\Form\AntispamField->setSessionData()
(vendor/madeyourday/contao-rocksolid-antispam/src/Form/AntispamField.php:125)
at MadeYourDay\RockSolidAntispam\Form\AntispamField->generate()
(vendor/contao/core-bundle/src/Resources/contao/library/Contao/Widget.php:660)
at Contao\Widget->generateWithError()
(vendor/madeyourday/contao-rocksolid-antispam/src/Resources/contao/templates/form_rocksolid_antispam.html5:3)
at include('/home/httpd/vhosts/kunde.ch/kunde.ch-c413/kunde.ch/vendor/madeyourday/contao-rocksolid-antispam/src/Resources/contao/templates/form_rocksolid_antispam.html5')
(vendor/contao/core-bundle/src/Resources/contao/library/Contao/TemplateInheritance.php:108)
at Contao\Widget->inherit()
(vendor/contao/core-bundle/src/Resources/contao/library/Contao/Widget.php:607)
at Contao\Widget->parse()
(vendor/contao/core-bundle/src/Resources/contao/forms/Form.php:248)
at Contao\Form->compile()
(vendor/contao/core-bundle/src/Resources/contao/classes/Hybrid.php:230)
at Contao\Hybrid->generate()
(vendor/contao/core-bundle/src/Resources/contao/forms/Form.php:90)
at Contao\Form->generate()
(vendor/contao/core-bundle/src/Resources/contao/library/Contao/Controller.php:621)
at Contao\Controller::getContentElement(object(ContentModel))
(vendor/contao/news-bundle/src/Resources/contao/classes/News.php:226)
at Contao\News->generateFiles(array('id' => '1', 'tstamp' => '1408000129', 'title' => 'kunde', 'alias' => 'kunde', 'language' => 'de', 'archives' => 'a:2:{i:0;s:1:"4";i:1;s:1:"6";}', 'format' => 'rss', 'source' => 'source_text', 'maxItems' => '10', 'feedBase' => 'http://www.kunde.ch/', 'description' => '', 'imgSize' => '', 'feedName' => 'kunde'))
(vendor/contao/news-bundle/src/Resources/contao/classes/News.php:105)
at Contao\News->generateFeedsByArchive('4')
(vendor/contao/news-bundle/src/Resources/contao/dca/tl_news_archive.php:480)
at tl_news_archive->generateFeed(object(DC_Table))
(vendor/contao/core-bundle/src/Resources/contao/drivers/DC_Table.php:201)
at Contao\DC_Table->__construct('tl_news_archive', array('tables' => array('tl_news_archive', 'tl_news', 'tl_news_feed', 'tl_content'), 'table' => array('Contao\TableWizard', 'importTable'), 'list' => array('Contao\ListWizard', 'importList')))
(vendor/contao/core-bundle/src/Resources/contao/classes/Backend.php:416)
at Contao\Backend->getBackendModule('news', null)
(vendor/contao/core-bundle/src/Resources/contao/controllers/BackendMain.php:168)
at Contao\BackendMain->run()
(vendor/contao/core-bundle/src/Controller/BackendController.php:49)
at Contao\CoreBundle\Controller\BackendController->mainAction()
(vendor/symfony/http-kernel/HttpKernel.php:163)
at Symfony\Component\HttpKernel\HttpKernel->handleRaw(object(Request), 1)
(vendor/symfony/http-kernel/HttpKernel.php:75)
at Symfony\Component\HttpKernel\HttpKernel->handle(object(Request), 1, true)
(vendor/symfony/http-kernel/Kernel.php:202)
at Symfony\Component\HttpKernel\Kernel->handle(object(Request))
(web/index.php:44)