counter: fix eticket server crash induced by old permission system and fix Selling permission

counter/models.py

@@ -478,6 +478,8 @@ class Selling(models.Model):
         return user.is_owner(self.counter) and self.payment_method != "CARD"
 
     def can_be_viewed_by(self, user):
+        if not hasattr(self, "customer"):  # Customer can be set to Null
+            return False
         return user == self.customer.user
 
     def delete(self, *args, **kwargs):

counter/views.py

@@ -1752,7 +1752,11 @@ class EticketPDFView(CanViewMixin, DetailView):
         from reportlab.graphics.barcode.qr import QrCodeWidget
         from reportlab.graphics import renderPDF
 
-        self.object = self.get_object()
+        if not (
+            hasattr(self.object, "product") and hasattr(self.object.product, "eticket")
+        ):
+            raise Http404
+
         eticket = self.object.product.eticket
         user = self.object.customer.user
         code = "%s %s %s %s" % (