Symfony version(s) affected: 4.4.9

Description

I'm using a childAdmin with Sonata
https://symfony.com/doc/master/bundles/SonataAdminBundle/reference/child_admin.html

When I'm on a playList/idPlaylist/video/create url and try to submit the form to create a video and add it to the playlist, I get Unable to get a property on a non-object..

Everything worked fined with symfony-form@4.4.8, but when I update to 4.4.9 I get an error.

Additional context