[great-users] Exception: Object handle is null

[Gabriele Trombetti]

Starting from a transformation which worked, I made some modifications. 
I modified the destination metamodel, upgraded the library in GReAT, 
changed the Root Element for the destination metamodel and modified some 
rules. I can't see any error in my transformation, but I get an 
exception on a rule which I didn't actually modify:

----
caption: "GME"
text: "Exception: Object handle is null"
----

The rule is simple:
get a child from the element from the source metamodel,
create a corresponding child from the element from the destination 
metamodel (CreateNew)
link the two with a cross-diagram association (also CreateNew)
put in output the same elements which I got from the input

If I remove the create-new-crosslink-association from the diagram, the 
exception disappears and all the transformation runs to the end. 
Unfortunately I need that cross link...
To my knowledge, the two elements which are to be linked with the 
crosslink DO exist, and there are no NULL references in the GReAT 
transformation.
The GReAT debugger says it's a UDM exception.

Anybody has an idea?
Thank you
Gabriele