Fixed a seg fault that occurs when trying to edit a control net when there is...

Fixed a seg fault that occurs when trying to edit a control net when there is not an active control net set. Fixed a seg fault that occurs when right clicking on a control network when it is the only item on the project tree. Fixes #5048, #5071.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce@7945 41f8697f-d340-4b68-9986-7bafba869bb8