Steps to reproduce

1. Log in to back office.
2. Go to Admin / Activity log.
3. In the Filters section expand 'Object class' section.
4. Select one of available option.
5. Apply the filter.

Actual result:

Filtering by object class causes an error 500.

Argument 1 passed to Ibexa\Bundle\ActivityLog\Controller\AbstractLogController::Ibexa\Bundle\ActivityLog\Controller\{closure}() must be of the type string, object given 

Expected result:

User can filter recent activity by object class.