Steps to reproduce:
1. Navigate to the Control Panel -> Custom Fields -> User
2. Add Custom Field with a key of "testGroups" and a "True/False", click Save
3. Navigate to Control Panel -> Users -> Users & Organizations -> add User
4. Scroll down to Custom Fields and inspect HTML of the Testgroups
Expected: The id attribute of Select is unique and does not contain null
Actual: The id attribute contains "null_null_"