Fatal when using a role contextual filter

@dawehner thinks this might be a duplicate of an existing issue, but I wanted to make sure to document it until we confirm.

Steps to reproduce

1. Install 8.x Standard.
2. Create a new view of users with a page display and path users-by-role.
3. Add a contextual filter for the user role.
   
4. Go to users-by-role/administrator. You get the fatal:
   
   Fatal error: Call to a member function label() on a non-object in /Applications/MAMP/htdocs/d8git/core/modules/user/lib/Drupal/user/Plugin/views/argument/RolesRid.php on line 23