fix js error you sometimes get when editing an appointment group

fixes CNVS-12558

test plan:
 * enable calendar2 and scheduler
 * go to calendar, then scheduler view
 * ensure you have an appointment group (if not, create one)
 * ensure appointment_group_id=null is *not* in the url
 * click the appointment group to view the calendar for it
 * appointment_group_id=x should be in the url
 * click "Go back to list"
 * appointment_group_id= should *not* be in the url
 * reload the page
 * try to edit the appointment group
 * it should work

Change-Id: Idc15a14a1957b8322b1f48ee577770072309b114
Reviewed-on: https://gerrit.instructure.com/33647
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>