BUG: fix the crash on scene close, see issue #3313

Parameter node was referenced from a node selector event handler in the process
of scene close. Since parameter node was deleted before the the node selectors
were updated, this led to invalid reference.

The fix eliminates references to parameter node from node selector event handlers,
and does this only when user initiates the processing.



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22309 3bd1e089-480b-0410-8dfb-8563597acbee