Add a build tool which does nullability checks

[felixbarny]

Other code quality tools might also be interesting, but I had bad experiences with most of them in terms of an overwhelming amount of false-positives.

[alvarolobato]

Findbugs usually works well, the problem is they have deprecated it in favor of JSR-305 which is not published and in status dormant. Not being published affects the licensing in some situations for non OSS software. I've seen many OSS projects use it, it should be fine.