Require a session to be present at all times, not just in pretix.presale views

src/pretix/multidomain/middlewares.py

@@ -67,6 +67,14 @@ class SessionMiddleware(BaseSessionMiddleware):
     a custom domain.
     """
 
+    def process_request(self, request):
+        super().process_request(request)
+
+        if not request.session.session_key:
+            # We need to create session even if we do not yet store something there, because we need the session
+            # key for e.g. saving the user's cart
+            request.session.create()
+
     def process_response(self, request, response):
         try:
             accessed = request.session.accessed