I do not at all.
Discussed this with thorben.
So...it is just a pointer...so order of deletion is not cruical.
If the graph is not "valid" any more,  the holder classes are also not valid any more.
But even if the destructor of the graph itself has been called, the nodeHolder object can
be deleted without trouble.
I could not think of a reason where this leads to trouble...
If the graph has been garbage collected and nodes sill exist it will obviously  crash
if someone tries to use functions of the node related to the graph.
But that's perfectly ok with my understanding of python.

(offline now for today )