Plugin misinterprets URL components

The ui-requests module uses the find user plugin in its 'create request' form. If an existing request is selected while the new request form is open (so that the URL looks something like http://localhost:8080/requests/view/f1f7a710-84e6-4965-ab68-afec194d3cd3?layer=create), then attempting to use the user plugin by clicking the Requester lookup link fails with the error No user found by id f1f7a710-84e6-4965-ab68-afec194d3cd3. The plugin is misinterpreting the item ID used by the requests module as a specified user ID, and hilarity a bad user experience ensues.

Conceptually, this seems related to the problem addressed in UIPFU-6.