fix: create/edit subscriber modal

[patriciareinoso]

Description

fix for #711 and #712

Issue: we cannot create/edit subscribers in network slices that contain more than 1 device group. We are not able to select an element of the dropdown.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.