slapos_accounting: do not crash if open order line is not linked to an instance

master/bt5/slapos_accounting/SkinTemplateItem/portal_skins/slapos_accounting/OpenSaleOrder_updatePeriod.py

@@ -58,8 +58,10 @@ if open_sale_order.getValidationState() == 'validated':
 
       hosting_subscription = open_order_line.getAggregateValue(portal_type='Hosting Subscription')
       instance_tree = open_order_line.getAggregateValue(portal_type='Instance Tree')
+
       assert current_start_date == hosting_subscription.HostingSubscription_calculateSubscriptionStartDate()
 
+      if instance_tree is not None:
         subscription_request = instance_tree.getAggregateRelatedValue(portal_type="Subscription Request")
         # Define the start date of the period, this can variates with the time.
         next_stop_date_delta = 0