qview no longer crashes when attempting to apply a stretch to a cube that is...

qview no longer crashes when attempting to apply a stretch to a cube that is still loading. Fixes #2117.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6310 41f8697f-d340-4b68-9986-7bafba869bb8