A working list cannot be deleted if it is assigned as a User's Restricted Access Working List. Trying to delete such a working list will result in the following error.
In Query Builder, run the following query to determine which User(s) have been assigned Restricted Access to the Working List, replacing xxxxx with the appropriate WORKING_LIST_ID:
select "Username" from security_users_vw where "User_ID" in (select security_users_id from security_user_wl where working_list_id=xxxxx)
Remove the working list from the Restricted Access Working List section for the User(s) indicated by the query. Once this has been done for all users, the working list can be successfully deleted.