Hello Stoyo,
Thanks for the sample that you have sent a long ago for the exception that occur on right clicking on the diagram. It was of exceptionally good help. But the case in which the code was crashing is still in there in the same of yours. Though I have made some changes which I think might causing the code crash. Here is the modification that I've made in the sample. I have changed the Property "Behavior" of the diagram to "Modify". I think this might be the cause of the problem. I request you to please go though the attached project and debug it I am right and in the case please suggest some solutions to the same. Here are the steps that are the key to raise this exception:

1. Select any node by left clicking on it.
2. Keeping the node selected, make selection in the empty area, by left clicking and dragging the mouse pointer.
3. Without leaving the left mouse button through which the selection we were creating, make the right click immediately.
4. The code will crash throwing Unhanded silverlight exception.

Thanks in advance

I am adding the code files both xaml and cs along with the post.