Steps to reproduce:

1. Create a test resource, like a Stack
2. Choose an Alerter, and add this to the Resource Blacklist
3. Delete the test Stack
4. Return to the Alerter: UI notes that there are alerts Blacklisted by # resource, but this resource(s) doesn't exist.
5. To confirm, click the Edit Resource button. The deleted Resource cannot be found in the list (because it's deleted).
6. (I only tested Blacklist, but I assume the same happens with a Whitelist)

No resources are enabled: