fix #5148 -- nbgrader "generate student version" crash; caused by cell_list and cells be temporarily out of sync for some reason (might be a race condition during notebook creation); this code change will make things robust in a natural way (just don't render the not-defined cell)
fix #5148 -- nbgrader "generate student version" crash; caused by cell_list and cells be temporarily out of sync for some reason (might be a race condition during notebook creation); this code change will make things robust in a natural way (just don't render the not-defined cell)

sagemathinc/smc,sagemathinc/smc,sagemathinc/smc,sagemathinc/smc