Subclass tree index: setting key to null for parent class obj 12072020 (#4433)
Subclass tree index: setting key to null for parent class obj 12072020 (#4433)

* setting key to null for tree index of property
that belongs to subclass of the class

* clean up

* adding returnKeyForValue
foam-framework/foam2,foam-framework/foam2,jacksonic/vjlofvhjfgm,foam-framework/foam2,jacksonic/vjlofvhjfgm,foam-framework/foam2,foam-framework/foam2,jacksonic/vjlofvhjfgm