Fix (very small, 1 page) Ultimate Relational Fields Plugin that goes with 'Ultimate Member' plugin

There is a small 1 page wordpress plugin called 'Ultimate Relational Fields' which allows a user with the Ultimate Member plugin (a very well resourced and documented wordpress 'member profiles' plugin), - to have their users select other users to display on their profile.

The small plugin is old and outdated, it works for me all the way through to selecting users, but when I save the profile, it doesn't save the data I chose to the profile anymore, and consequently it doesn't show that selection in the public view either, so it just needs a little update to go with the new version of Ultimate Member plugin.

It's a very small plugin, as you can see here:
https://github.com/plusplugins/um-relational-fields/blob/master/plugin.php

....and this is the main plugin Ultimate Member:
https://wordpress.org/plugins/ultimate-member/
https://ultimatemember.com/.

Here is a handy video of how it should work: https://www.youtube.com/watch?v=yk5zPfK8zfE
Thanks