Modified ExportControlNetWorkOrder::setupExecution() so that it no longer seg...

Modified ExportControlNetWorkOrder::setupExecution() so that it no longer seg faults when the user attempts to export a control network from the file menu.  Fixes #4816

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce_FY17-Sprint1@7625 41f8697f-d340-4b68-9986-7bafba869bb8