You have a bug in CRM 2016.  Tried to submit on connect site and got error.

https://connect.microsoft.com/dynamicssuggestions

Service Activities

1. You get null when you use Form Assistant to select people resources.  Resources also show as loading indefinitely.
   
   
   
   1. Noted that service selection rules keep switching to Custom after saving rules
   
   
   2. Was able to replicate this in TestOrg DB – looks like a bug