order_paymentevent.reference may not be NULL about django-oscar-paypal HOT 2 CLOSED

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class PaymentDetailsView(views.PaymentDetailsView):

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    def handle_payment(self, order_number, total_incl_tax, **kwargs):
        # Make submission to PayPal.
        print("handle_payment", order_number, total_incl_tax, kwargs)
        try:
            # Using authorization here (two-stage model).  You could use sale to
            # perform the auth and capture in one step.  The choice is dependent
            # on your business model.
            facade.authorize(order_number,
                             total_incl_tax,
                             kwargs['bankcard'],
                             kwargs['billing_address'])
        except facade.NotApproved, e:
            # Submission failed
            raise exceptions.UnableToTakePayment(e.message)

        # Record payment source and event
        source_type, is_created = models.SourceType.objects.get_or_create(name='PayPal')
        source = models.Source(source_type=source_type,
                               currency='GBP',
                               amount_allocated=total_incl_tax)
        self.add_payment_source(source)
        self.add_payment_event('Authorised', total_incl_tax, SOMETHINGSOMETHINGSOMETHING_TRANSACTIONID)