Added a presence check before removing a list element. Addresses a 500 in unassigned review form POST.

 - Legacy-Id: 14642

Author: levkowetz

ietf/group/views.py

@@ -1485,7 +1485,10 @@ def manage_review_requests(request, acronym, group_type=None, assignment_status=
             while head_of_rotation in assignments_by_person:
                 review_req = assignments_by_person[head_of_rotation]
                 assign_review_request_to_reviewer(request, review_req, review_req.form.cleaned_data["reviewer"],review_req.form.cleaned_data["add_skip"])
-                reqs_to_assign.remove(review_req)
+                if review_req in reqs_to_assign:
+                    # XXX FIXME: It's not clear how we could end up here, but
+                    # it has happened.  See traceback email dated 14 Feb 2018
+                    reqs_to_assign.remove(review_req)
                 head_of_rotation = reviewer_rotation_list(group)[0]
 
             for review_req in reqs_to_assign: