500 Internal Server Error response from helix-rest when updating cluster config

[micahstubbs]

Describe the bug

When updating a cluster config mapField value from Helix UI, helix-rest returns a 500 Internal Server Error response.

To Reproduce

1. in Helix UI, visit the cluster config page
2. open the network tab of the browser dev tools
3. Click the lock to unlock the fields and enter edit mode
4. locate a Map Field and edit its value
5. notice the 500 error from helix-rest in the browser dev tools network tab

Expected behavior

The POST request to update the config should succeed and helix-rest should respond with a 200.