Fixed PathListingWidget errors when "/" path is invalid.

Invalid root paths occur quite frequently in the SceneHierarchy editor when an invalid filename has been entered in a SceneReader or AlembicReader. Those nodes will still display an error, but the SceneHierarchy now won't output a confusing stacktrace which distracts people from the root cause.

Fixes #528.
Merge pull request #570 from johnhaddon/pathListingWidgetFix

Fixed PathListingWidget errors when "/" path is invalid.