Protect against graceperiodjson being null.

r.don

cms/djangoapps/models/settings/course_grading.py

 from xmodule.modulestore import Location
 from contentstore.utils import get_modulestore
-import datetime
 import re
 from util import converters
-import time
 
 
 class CourseGradingModel:
@@ -145,11 +143,15 @@ class CourseGradingModel:
     def update_grace_period_from_json(course_location, graceperiodjson):
         """
         Update the course's default grace period. Incoming dict is {hours: h, minutes: m} possibly as a 
-        grace_period entry in an enclosing dict.
+        grace_period entry in an enclosing dict. It is also safe to call this method with a value of
+        None for graceperiodjson.
         """
         if not isinstance(course_location, Location):
             course_location = Location(course_location)
 
+        # Before a graceperiod has ever been created, it will be None (once it has been
+        # created, it cannot be set back to None).
+        if graceperiodjson is not None:
             if 'grace_period' in graceperiodjson:
                 graceperiodjson = graceperiodjson['grace_period']
 
@@ -171,7 +173,7 @@ class CourseGradingModel:
         index = int(index)        
         if index < len(descriptor.raw_grader):
             del descriptor.raw_grader[index]
-            # force propagation to defintion
+            # force propagation to definition
             descriptor.raw_grader = descriptor.raw_grader
             get_modulestore(course_location).update_item(course_location, descriptor.definition['data'])