Can't add a just-removed association on the Edit Supply Partner screen

During the exploratory tests, I noticed that if I delete an association, add it and click "Update Supply Partner" button, an error appears.

Reproduction steps:

1. Log in and go to Administration > Supply Partner.

2. Click the "Edit" button next to the supply partner.

3. Remove an association.

4. Add again the same association to the supply partner.

5. Click the "Update Supply Partner" button.

6. An error appears that the given program and supervisory node already exist as an association.

Expected results:

• Information about successful update should appear.

• User should be able to add again an association.