Judging by the stack trace, he has a reference with a null url. This almost always violates spec, unless the reference is consumed rather than used. Either way, it's obvious that ExportedClassFolderReferenceResolver should at least guard against the possibility of a null reference url.
I'm committing a simple guard check. The logic here is that if the url is null, it cannot possibly be an ExportedClassFolderReferenceResolver reference, since all such references have a url beginning with module:/org.jboss.ide.eclipse.as.wtp.core.vcf.exportedClassFolders
A null reference url cannot be an ExportedClassFolderReferenceResolver. The null check will ensure this method returns false.