slapos_subscription_request: add assert when creating Open Order

master/bt5/slapos_subscription_request/SkinTemplateItem/portal_skins/slapos_subscription_request/SubscriptionRequest_createOpenSaleOrder.py

 portal = context.getPortalObject()
 subscription_request = context
 
+assert subscription_request.getAggregate('') != ''
+
 activate_kw = None
 
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