See Juergen's comment on the bug entry. He does have a point. getObjectType should never through a NPE even with a prototype coming in as the last name on the interceptors list, as the targetsource will default to the specail empty target source instance, which will just return null for the type. Now this means you should get a null result from the getObjectType, but that's legal.
Can you try to confirm where and how you're getting the NPE? Perhaps the easiest thing to do is to do is put a breakpoint on the code, and attach to the servlet engine. I think things are not what they seem.
Now, there's the question of whether the code should still try to do more to return a valid object type in this case (instead of null), which it could, and I think it should...