I was testing the new messaging's system with Clean theme in Moodle '3.6.1 (Build: 20181205)'.
It was running correctly when I tried to add new users to my contacts and an error appeared.
I had tested with Clean and Boost themes.
It's possible that in the latest release from Moodle 3.6.1 it's working. I don't know if It's my fault or it's a bug from Messaging system.