An existing subscriber is not allowed to edit to use another slice if another slice has more than one DG.

[gatici]

Describe the bug

An existing subscriber can not start to use another slice if it has more than one device groups (DG).

To Reproduce

Create a slice.
Create a subscriber using this slice.
Create a second slice.
Add another DG to second slice.
Try to edit subscriber to select second slice and one of the second slice's DG's.
None of the DG's are allowed to select.

Expected behavior

A subscriber should be edited to select another slice and one of another slice's DG.

Screenshots

Slice2 has 2DGs.

Slice2 has 1 DG.

Subscriber is created using slice deee.

When we select slice2, none of the DG's are allowed to select.

We can successfully select the DG from slice3 and update the subscriber.

Logs

Environment

• Charm / library version (if relevant): sdcore-nms-k8s
• Juju version (output from juju --version): 3.6.1
• Cloud Environment: 1.31
• Kubernetes version (output from kubectl version --short):

Additional context