JBIDE-17947 - Java Model Exception occurs while typing

The real change is https://github.com/xcoulon/jbosstools-webservices/compare/JBIDE-17947?expand=1#diff-123729c5e804834c2292401bb76ddd7aR1057

Other changes are:
- a JUnit test to reproduce the case (the method did not exist when the validation was called, because when the signature changes, JDT removes the old method and creates another one in its Java Model)
- added a few null-checks to avoid subsequent NPEs