API: fix event creation sales-channel-migration (#4949)

* API: fix event creation sales-channel-migration

* do not allow sales_channels to be None

* fix wrong validation error-key

src/pretix/api/serializers/__init__.py

@@ -81,6 +81,13 @@ class SalesChannelMigrationMixin:
 
     def to_internal_value(self, data):
         if "sales_channels" in data:
+            if data["sales_channels"] is None:
+                raise ValidationError({
+                    "sales_channels": [
+                        "The legacy attribute 'sales_channels' cannot be set to None, it must be a list."
+                    ]
+                })
+
             prefetch_related_objects([self.organizer], "sales_channels")
             all_channels = {
                 s.identifier for s in
@@ -89,7 +96,7 @@ class SalesChannelMigrationMixin:
 
             if data.get("all_sales_channels") and set(data["sales_channels"]) != all_channels:
                 raise ValidationError({
-                    "limit_sales_channels": [
+                    "all_sales_channels": [
                         "If 'all_sales_channels' is set, the legacy attribute 'sales_channels' must not be set or set to "
                         "the list of all sales channels."
                     ]
@@ -109,6 +116,7 @@ class SalesChannelMigrationMixin:
             else:
                 data["all_sales_channels"] = False
                 data["limit_sales_channels"] = data["sales_channels"]
+
             del data["sales_channels"]
 
         if data.get("all_sales_channels"):