When configuring the ASSIGN RIGHTS activity in Workflow for Cloud, could there be a check when validating the workflow that the Trustee exists?

You can pick the named user names for the Trustee, but when assigning a group you have to hand type it as the group names do not appear in the Trustee list. 

If you leave the trustee blank and validate, you will get a message that there was no Trustee selected. This is my way of saying it should be easy to add because part of the mechanism is already there :)