Grav v1.6.21 breaks on PHP 7.4? (E_NOTICE - Trying to access array offset on value of type null)

[ulfben]

I'm running Grav v1.6.21 on Ubuntu 18.04. After upgrading PHP to 7.4.3 the site broke - when loading the front page I get:

E_NOTICE - Trying to access array offset on value of type null

The log says:
[2020-02-20 21:51:58] grav.CRITICAL: Trying to access array offset on value of type null - Trace:
#0 /var/www/infoscreen/system/src/Grav/Common/Debugger.php(352): Whoops\Run->handleError()
#1 /var/www/infoscreen/vendor/erusev/parsedown-extra/ParsedownExtra.php(241): Grav\Common\Debugger->deprecatedErrorHandler()
#2 /var/www/infoscreen/vendor/erusev/parsedown/Parsedown.php(206): ParsedownExtra->blockSetextHeader()
#3 /var/www/infoscreen/vendor/erusev/parsedown/Parsedown.php(39): Parsedown->lines()
#4 /var/www/infoscreen/vendor/erusev/parsedown-extra/ParsedownExtra.php(46): Parsedown->text()
#5 /var/www/infoscreen/system/src/Grav/Common/Page/Page.php(849): ParsedownExtra->text()
#6 /var/www/infoscreen/system/src/Grav/Common/Page/Page.php(692): Grav\Common\Page\Page->processMarkdown()
#7 /var/www/infoscreen/system/src/Grav/Common/Twig/Twig.php(366): Grav\Common\Page\Page->content()
#8 /var/www/infoscreen/system/src/Grav/Common/Service/OutputServiceProvider.php(28): Grav\Common\Twig\Twig->processSite()
#9 /var/www/infoscreen/vendor/pimple/pimple/src/Pimple/Container.php(118): Grav\Common\Service\OutputServiceProvider->Grav\Common\Service{closure}()
#10 /var/www/infoscreen/system/src/Grav/Common/Processors/RenderProcessor.php(28): Pimple\Container->offsetGet()
#11 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\RenderProcessor->process()
#12 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#13 /var/www/infoscreen/system/src/Grav/Common/Processors/DebuggerAssetsProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle()
#14 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\DebuggerAssetsProcessor->process()
#15 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#16 /var/www/infoscreen/system/src/Grav/Common/Processors/PagesProcessor.php(69): Grav\Framework\RequestHandler\RequestHandler->handle()
#17 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\PagesProcessor->process()
#18 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#19 /var/www/infoscreen/system/src/Grav/Common/Processors/TwigProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle()
#20 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\TwigProcessor->process()
#21 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#22 /var/www/infoscreen/system/src/Grav/Common/Processors/AssetsProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle()
#23 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\AssetsProcessor->process()
#24 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#25 /var/www/infoscreen/system/src/Grav/Common/Processors/SchedulerProcessor.php(29): Grav\Framework\RequestHandler\RequestHandler->handle()
#26 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\SchedulerProcessor->process()
#27 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#28 /var/www/infoscreen/system/src/Grav/Common/Processors/BackupsProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle()
#29 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\BackupsProcessor->process()
#30 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#31 /var/www/infoscreen/system/src/Grav/Common/Processors/TasksProcessor.php(59): Grav\Framework\RequestHandler\RequestHandler->handle()
#32 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\TasksProcessor->process()
#33 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#34 /var/www/infoscreen/system/src/Grav/Common/Processors/RequestProcessor.php(52): Grav\Framework\RequestHandler\RequestHandler->handle()
#35 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\RequestProcessor->process()
#36 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#37 /var/www/infoscreen/system/src/Grav/Common/Processors/ThemesProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle()
#38 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\ThemesProcessor->process()
#39 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#40 /var/www/infoscreen/system/src/Grav/Common/Processors/PluginsProcessor.php(30): Grav\Framework\RequestHandler\RequestHandler->handle()
#41 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\PluginsProcessor->process()
#42 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#43 /var/www/infoscreen/system/src/Grav/Common/Processors/InitializeProcessor.php(95): Grav\Framework\RequestHandler\RequestHandler->handle()
#44 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\InitializeProcessor->process()
#45 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#46 /var/www/infoscreen/system/src/Grav/Common/Processors/DebuggerProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle()
#47 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\DebuggerProcessor->process()
#48 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#49 /var/www/infoscreen/system/src/Grav/Common/Processors/ErrorsProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle()
#50 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\ErrorsProcessor->process()
#51 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#52 /var/www/infoscreen/system/src/Grav/Common/Processors/LoggerProcessor.php(48): Grav\Framework\RequestHandler\RequestHandler->handle()
#53 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\LoggerProcessor->process()
#54 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#55 /var/www/infoscreen/system/src/Grav/Common/Processors/ConfigurationProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle()
#56 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\ConfigurationProcessor->process()
#57 /var/www/infoscreen/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle()
#58 /var/www/infoscreen/system/src/Grav/Common/Grav.php(272): Grav\Framework\RequestHandler\RequestHandler->handle()
#59 /var/www/infoscreen/index.php(54): Grav\Common\Grav->process()
#60 {main} [] []

All plugins are up-to-date, and I've tried disabling them all but the problem persist.

Is this an issue on my end, and if so, how do I trouble-shoot?

[ulfben]

Sorry! Posted on the wrong repo. Can't see a way to delete this so I'll just close it.