Attempting to send a tag to the clinic that doesn't have an assigned serial number generates a 500 error

[dylanm312]

Describe the bug
On the mainline page, if you try to send a tag to the clinic that never got a serial number assignment on the Tag Assignment page, the AJAX request will return a 500 error (visible in the browser console). These tags show up as "S/N not found" in the "Send to Clinic" dropdown.

To Reproduce

1. Go to the mainline page
2. Place some tags in a zone (in the real world), and select that zone using the dropdown on the mainline page
3. Click the "Send to Clinic" button
4. Select a tag without a serial number assigned (shows up as "S/N not found" in the dropdown)
5. Press "Send to Clinic"
6. Observe the 500 error in the browser console. Additionally, the modal doesn't close.

Expected behavior
The user should either:
A. Receive an error message stating that the selected tag needs to be assigned a serial number first, or
B. Not be able to initiate such a request for a non-assigned tag in the first place, or
C. Be able to send the tag anyway, possibly with a warning that the tag is not assigned.