As a work-around for 6.6.0 I got away with defining two String properties for my POJO used to mock the unit tests. The two properties are “choices” and “selection”.
This works because the crash happens when the FormTester is trying to get access to all of the form fields values and attempts to resolve those properties against the backing POJO for the PropertyModel.
Upgrading to 6.7.0 is not an option for us because of
WICKET-5147 "WicketTester MockHttpRequest.getCookies very slow / OutOfMemory" which is fixed in 6.8.0 (scheduled for this Friday, May/17/2013).