Session might not exist for rebuild theme registry warning

Problem/Motivation

When using the " Rebuild the theme registry on every page load" feature in combination with "Access developer information" permission for anonymous users, I occasionally run into an Exception caused by the theme rebuild info message. In those cases, the session does not exist on the request - maybe due to sub-requests or some request altering code.

Error: Call to a member function has() on null in Drupal\devel\EventSubscriber\ThemeInfoRebuildSubscriber->triggerWarningIfNeeded() (line 99 of /srv/www/htdocs/eposs-dev.w21.it/html/modules/dev/devel/src/EventSubscriber/ThemeInfoRebuildSubscriber.php)

Proposed resolution

Check if the session exists before trying to use it in Drupal\devel\EventSubscriber\ThemeInfoRebuildSubscriber::triggerWarningIfNeeded:
if ($session && !$session->has($this->notificationFlag)) { /* ... */ }

Remaining tasks

Review patch.