V12: Creating System User Bug/Error: AttributeError: 'NoneType' object has no attribute 'replace'

Hi, can anyone help me in this issue I am facing? My colleagues have successfully created more than 40 employees and now it is my turn to create “User” accounts for them. When I am trying to ‘link’ user account with the employees, I received the error in the screenshot below.

P.S. I am using v12.04.
Previously, while I am learning to use v11, I never had this problem.