5.0.4 Beta - TypeError: array_key_exists() on login

Upgraded to 5.0.4 Beta on my dev site this morning.

Now, upon log in, I'm getting the following error. A refresh often clears it, and you can navigate freely but then it will pop up again and cleared with a refresh of the page.

TypeError: array_key_exists(): Argument #2 ($array) must be of type array, null given (0)

#0 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Output/Plugin/Theme.php(98): array_key_exists()

#1 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Theme/Theme.php(4174): IPS\Output\Plugin\Theme::runPlugin()

#2 [internal function]: IPS\Theme::IPS\{closure}()

#3 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Theme/Theme.php(4126): preg_replace_callback()

#4 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Theme/Theme.php(4047): IPS\Theme::_compileTemplate()

#5 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Theme/Theme.php(3811): IPS\Theme::compileTemplate()

#6 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Theme/Theme.php(1149): IPS\Theme::makeProcessFunction()

#7 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/applications/cms/sources/Templates/Templates.php(1260): IPS\Theme::compileCustomCss()

#8 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Patterns/ActiveRecord.php(360): IPS\cms\Templates->get__file_object()

#9 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/applications/cms/sources/Pages/Page.php(1131): IPS\Patterns\ActiveRecord->__get()

#10 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/applications/cms/sources/Pages/Page.php(2080): IPS\cms\Pages\Page->getIncludes()

#11 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/applications/cms/modules/front/pages/page.php(122): IPS\cms\Pages\Page->output()

#12 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/applications/cms/modules/front/pages/page.php(49): IPS\cms\modules\front\pages\page->view()

#13 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Dispatcher/Controller.php(139): IPS\cms\modules\front\pages\page->manage()

#14 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/system/Dispatcher/Dispatcher.php(169): IPS\Dispatcher\Controller->execute()

#15 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/index.php(16): IPS\Dispatcher->run()

#16 {main}

Backtrace

#0 /var/www/vhosts/cheersandgears.com/dev.cheersandgears.com/init.php(827): IPS\Log::log()

#1 [internal function]: IPS\IPS::exceptionHandler()

#2 {main}