Fix invitation handling in mutation

aleksis/core/schema/person.py

@@ -598,6 +598,8 @@ class SendAccountRegistrationMutation(PersonAddressMutationMixin, graphene.Mutat
     @classmethod
     @transaction.atomic
     def mutate(cls, root, info, account_registration: AccountRegistrationInputType):
+        invitation = None
+
         if account_registration["invitation_code"]:
             try:
                 invitation = PersonInvitation.objects.get(key=account_registration["invitation_code"])