I don't know this code, but since pushViolations() and iterableFromConstraintViolations() are both public methods, under the principle of being conservative about what you send and liberal about what you receive, it seems like iterableFromConstraintViolations() should return an empty iterable when given a null (since whatever calls it probably expects to be able to iterate) and pushViolations() should do the null check.
https://gwt-code-reviews.appspot.com/1826803/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
