BUG: fixed bug in case the user does not select vertebral levels.
BUG: fixed bug in case the user does not select vertebral levels.

3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox