If a NS includes 2 DG just after slice creation, subscriber could not be created using this slice as DG could not be selected

[gatici]

Describe the bug

Using NMS UI, if we create an NS including 2 DG just after slice creation, it is not possible to create a subscriber using this slice.
The reason why Device group in the subscriber could not be selected.

To Reproduce

1. Deploy SD-Core from 1.6/edge channel
2. Create a slice
3. Add a second DG
4. Try to create a subscriber using this slice
5. Device group can not be selected and create button is stay in disabled.

Expected behavior

NMS should allow us to select one of the DG while creating subscriber.

Screenshots

None of DGs are allowed to select.

Logs

Environment

• Charm / library version (if relevant): sdcore-nms-k8s
• Juju version (output from juju --version): 3.6.1
• Cloud Environment: Microk8s: 1.31
• Kubernetes version (output from kubectl version --short):

Additional context