Mark the code segment with a red circle, assign a value to the highnode, and print the diagram value of the highnode field as null.
If Diagram.findPartAt returns an instance of a Node, the value of Node.diagram (i.e. the GraphObject.diagram property) will be that Diagram. Otherwise it could not find the Node at that point in the Diagram.
If you could give me some details for how to reproduce the problem, I can try it.