So there are 2 different concerns we should handle:

1) references pointing to nodes from the source ws which have not been cloned (outside the graph) - I agree with the above that an exception would be appropriate
2) references which *are not of the type*: ```NodeKeyReference```. We have (still) ```UUIDReference``` and ```StringReference```. The current code just "keeps" them in the cloned property as is, but I'm not sure how this should be handled. Actually I'm not sure when instances of the latter 2 classes can show up internally.