Fix handling of session create

src/pretix/storefrontapi/endpoints/checkout.py

@@ -216,21 +216,22 @@ class CheckoutSessionSerializer(serializers.ModelSerializer):
             for p in payments
         ]
 
-        steps = get_steps(
-            self.context["event"],
-            cartpos,
-            getattr(checkout, "invoice_address", None),
-            checkout.session_data,
-            total,
-        )
         d["steps"] = {}
-        for step in steps:
-            applicable = step.is_applicable()
-            valid = not applicable or step.is_valid()
-            d["steps"][step.identifier] = {
-                "applicable": applicable,
-                "valid": valid,
-            }
+        if cartpos:
+            steps = get_steps(
+                self.context["event"],
+                cartpos,
+                getattr(checkout, "invoice_address", None),
+                checkout.session_data,
+                total,
+            )
+            for step in steps:
+                applicable = step.is_applicable()
+                valid = not applicable or step.is_valid()
+                d["steps"][step.identifier] = {
+                    "applicable": applicable,
+                    "valid": valid,
+                }
 
         return d
 

src/pretix/storefrontapi/steps.py

@@ -75,12 +75,12 @@ class PaymentStep(CheckoutStep):
         r.event = self.event
         r.organizer = self.event.organizer
         self.cart_session.setdefault("fake_request", {})
-        cart_id = self.cart_positions[0].cart_id
+        cart_id = self.cart_positions[0].cart_id if self.cart_positions else None
         r.session = UserDict(
             {
                 f"current_cart_event_{self.event.pk}": cart_id,
                 "carts": {cart_id: self.cart_session},
-            }
+            } if cart_id else {}
         )
         r.session.session_key = cart_id
         return r