I'm seeing a simliar issue but without deleting the admin user John. The steps to reproduce are:
1. Login as John
2. Add a new user using the admin page
3. Delete the newly added user
4. Logout
5. Login as John -> Results in NPE

This might be covered by a different jira but I was not able to find one.

This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators
For more information on JIRA, see: http://www.atlassian.com/software/jira