There have been reports of the following errors in the error log coming from the Recommendation Bundle (SessionBackup listener):

PHP Fatal error: Uncaught exception 'Symfony\\Component\\Debug\\Exception
FatalErrorException' with message 'Error: Call to a member function isStarted() on null' in /xxx/vendor/ezsystems/recommendation-bundle/EventListener/SessionBackup.php:28\nStack trace:\n#0

It seems that in some circumstances $request->getSession() might return null, which causes the issue above.

Steps to reproduce
I was not able to reproduce the issue on a clean installation of eZ Platform.