5.5.0-dev - Trying to get property 'language' of non-object

[kat05]

On a Manticore RocketLauncher Joomla 3.9.10 install I installed the 5.5.0-dev package.
When I refresh the frontend, I am getting an error:

Whoops\Exception\ErrorException thrown with message "Trying to get property 'language' of non-object"

Stacktrace:
#14 Whoops\Exception\ErrorException in ../libraries/gantry5/classes/Gantry/Framework/Theme.php:174
#13 Gantry\Component\Whoops\SystemFacade:handleError in ../libraries/gantry5/classes/Gantry/Framework/Theme.php:174
#12 Gantry\Framework\Theme:init in ../libraries/gantry5/classes/Gantry/Component/Theme/AbstractTheme.php:67
#11 Gantry\Component\Theme\AbstractTheme:__construct in ../templates/rt_manticore/includes/theme.php:28
#10 plgSystemGantry5:{closure} in ../libraries/gantry5/vendor/pimple/pimple/src/Pimple/Container.php:118
#9 Pimple\Container:offsetGet in ../plugins/system/gantry5/gantry5.php:274
#8 plgSystemGantry5:onAfterRouteSite in ../plugins/system/gantry5/gantry5.php:95
#7 plgSystemGantry5:onAfterRoute in ../libraries/joomla/event/event.php:70
#6 JEvent:update in ../libraries/joomla/event/dispatcher.php:160
#5 JEventDispatcher:trigger in ../libraries/src/Application/BaseApplication.php:108
#4 Joomla\CMS\Application\BaseApplication:triggerEvent in ../libraries/src/Application/CMSApplication.php:1122
#3 Joomla\CMS\Application\CMSApplication:route in ../libraries/src/Application/SiteApplication.php:796
#2 Joomla\CMS\Application\SiteApplication:route in ../libraries/src/Application/SiteApplication.php:218
#1 Joomla\CMS\Application\SiteApplication:doExecute in ../libraries/src/Application/CMSApplication.php:196
#0 Joomla\CMS\Application\CMSApplication:execute in ../index.php:49