Avoid multiple call to Admin::getObject and ModelManager::find

[VincentLanglet]

Subject

This is BC and supposed to reduce the amount of query.

assertObjectExists calls getObject which calls find on the ModelManager and execute a query.
Most of the time we assert the object exist then we're getting the object again.
If the assertion return the object, we can avoid the second call.

Changelog

### Changed
- Reduced number of ModelManager::find calls