When an agent is added to the customer list, it should not show a 'check' icon to indicate that the agent is successfully added, it should throw an error instead.

Summary

When heading to project > Customers, users
As agents should not be able to be added to the customer list, instead of showing a  icon after adding an agent, we should instead add an error message such as :

The user 'XXX' is an agent. Agents cannot be added to the customer list.

This will definitely avoid any confusions on our customer's end.