Steps to reproduce

1. Install Ibexa DXP
2. Create a user under the editor group
3. Create content by a created user
4. Delete created user
5. Login to admin and go to Content -> Content structure
6. Check the request to graphQL in the dev tool 

Result

{"errors":[{"debugMessage":"Could not find 'Content' with identifier array (
   'id' => 64,
   'languages' => 
      array (
         0 => 'eng-GB', 
      ),
   'versionNo' => NULL,
)",
"message":"Internal server Error" ....

Expected result

Content belonging to a removed user should not cause errors in graphQL