I have a third party software component that brings in Guice, and it looks like inner classes used in Guice Provider's wind up getting wired in as null when they come out of the Spring reload agent class loader.
Are you reloading by turning on the reloading support under the tc server settings in STS? I'm just aware that STS still includes quite an old level of the reloading agent. (1.0.1 I think, when it is actually up to 1.0.6). The latest version is here:
You could take that version and patch up your tc server plugin (put it in your STS com.springsource.sts.server.tc.reloading_126.96.36.199 205071000-RELEASE/lib folder) - or copy and modify the launch configuration you are using, pointing it at this jar rather than the one it will currently include.