Fix bug in nbgrader list removal
Fix bug in nbgrader list removal

ellisonbg/nbgrader,jupyter/nbgrader,MatKallada/nbgrader,modulexcite/nbgrader,jhamrick/nbgrader,alope107/nbgrader,jupyter/nbgrader,alope107/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,modulexcite/nbgrader,MatKallada/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,dementrock/nbgrader,jupyter/nbgrader,dementrock/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,ellisonbg/nbgrader,jupyter/nbgrader,jhamrick/nbgrader