JBIDE-18344 - revert previous change to AbstractJREComposite, use simple...

...r solution of setting selected page
Theoretically yes but realistically no, since we only pass in 2 page id's:  the one for exec-env or installed jre's.  Since these are part of base eclipse, it seems very unlikely they'd ever be null.  I suppose some client could subclass and pass in an invalid node, but that's also very unlikely, since this composite's entire purpose is to link to exec-env / jre pages.