Can't update a referenced field after the reference was deleted

[7hibault]

Prerequisites

• Are you running the latest version of ng-admin? Yes, running the ng-admin-demo.
• Did you check the documentation? Yes.
• Did you perform a cursory search to see if your bug or enhancement is already reported? Yes.

Description

When an entity contains a reference to another entity, deleting the referenced entity makes it no longer possible to edit the referenced field.

Steps to Reproduce

1. Run the ng-admin-demo
2. Go to Catalog > Posters
3. Pick any poster, click edit and check that you can update the Category field
4. Go to Catalog > Categories
5. Pick any category and delete it
6. Go to Catalog > Posters
7. Pick a poster that previously had the category you just deleted and click edit
8. Try to update the Category field

Expected behavior: The field should be editable

Actual behavior: The field can no longer be updated