If a Service Desk is disabled it doesn't show up in the enable drop down

Summary

When a Service Desk is disabled it no longer shows up in the drop-down for enabling as per the attached screenshot.

Steps to Reproduce

1. Enable Service Desk for a project.
2. Disable it.
3. Attempt to enable it again.

Expected Results

It appears in the drop down.

Actual Results

It does not.

Workaround

It can be enabled from the Project Admin section.