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
The problem is that as-you-type validation works in a separate job. It may happen that this method exists in the time of this null checking but won't when you do NodeFinder.perform().
So a core exception still can be thrown here. There is not much you can do actually. You should just be ready to get a Core Exception and handle it properly.