fix NoneType has no get_comment_group()
fix NoneType has no get_comment_group()

lino-framework/lino,lino-framework/lino,lsaffre/lino,lsaffre/lino,lsaffre/lino,lino-framework/lino,lino-framework/lino,lsaffre/lino,lsaffre/lino,lino-framework/lino