When using the getWidget() method in a Tool constructor, cannot assume a valid...

When using the getWidget() method in a Tool constructor, cannot assume a valid MosaicSceneWidget value.  Tools can be constructed with a Null MosaicSceneWidget.  This was causing a seg fault when accessing the qmos help.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce_FY17-Sprint1@7652 41f8697f-d340-4b68-9986-7bafba869bb8