fix: Set billing starts at to reasonable default for API usage notifi…

…cations (#4054)

api/organisations/views.py

@@ -2,6 +2,7 @@
 from __future__ import unicode_literals
 
 import logging
+from datetime import timedelta
 
 from app_analytics.influxdb_wrapper import (
     get_events_for_organisation,
@@ -324,6 +325,10 @@ def get_queryset(self):
         billing_starts_at = subscription_cache.current_billing_term_starts_at
         now = timezone.now()
 
+        # Handle case where billing dates are not set by
+        # defaulting to something as a reasonable default.
+        billing_starts_at = billing_starts_at or now - timedelta(days=30)
+
         month_delta = relativedelta(now, billing_starts_at).months
         period_starts_at = relativedelta(months=month_delta) + billing_starts_at