If the className attribute is not included with the <Realm> tag in server.xml, the server dies with a cryptic NullPointerException. It would be nice if it produced a more meaningful error message (like "Missing className attribute for <Realm> tag").
Agreed, a more meaningful error message would be nice. However, this is an enhancement request.
Note that this bug affects you if you accidently spell className classname, so it can be pretty difficult to find.
This has been fixed in 7.0.5 and will be included in 7.0.6 onwards.