When an element is associated with another element or diagram, a reference node
(see attached screenshot) is created under the element node. This reference node
is never created in the Beta 3 build. It's created in the latest daily builds
but sometimes it's not created and sometimes the node was created but lost after
a while.

Repro steps:
- create a uml project
- create 2 activity diagrams
- create and name a couple of invocation elements on each diagram
- from one of the element on the 1st diagram, right click and select "Associate
With..." from the context menu
- from the "Associate" dialog, enter a string that matches the 2nd diagram name
and click on Find
- from the result, select the 2nd diagram from the Search Result table
- check the "Automatically Set the Navigate on Double-Click Option" checkbox
- click on "Associate" button
- close the dialog
- expand the element, the reference node should be there