Exceptions in EMF-Frag editor when selecting objects

[markus1978]

Extensions and objects with indexed value sets cause exceptions when clicked in the EMF editor.

[markus1978]

1. cause: [ ] objects contained in a indexed value set of an indexed containment reference have no container.
2. cause: [ ] when an object is clicked, EMF checks for containment in all references of the container. Causes #contains on value sets.
3. cause: [x] EMF asks for the containment feature when a child is clicked. This causes null pointer exception for extensions, which have no containment feature.