API: Fix crash PRETIXEU-AN5

src/pretix/api/serializers/__init__.py

@@ -88,16 +88,20 @@ class SalesChannelMigrationMixin:
             }
 
             if data.get("all_sales_channels") and set(data["sales_channels"]) != all_channels:
-                raise ValidationError(
-                    "If 'all_sales_channels' is set, the legacy attribute 'sales_channels' must not be set or set to "
-                    "the list of all sales channels."
-                )
+                raise ValidationError({
+                    "limit_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."
+                    ]
+                })
 
             if data.get("limit_sales_channels") and set(data["sales_channels"]) != set(data["limit_sales_channels"]):
-                raise ValidationError(
-                    "If 'limit_sales_channels' is set, the legacy attribute 'sales_channels' must not be set or set to "
-                    "the same list."
-                )
+                raise ValidationError({
+                    "limit_sales_channels": [
+                        "If 'limit_sales_channels' is set, the legacy attribute 'sales_channels' must not be set or set to "
+                        "the same list."
+                    ]
+                })
 
             if data["sales_channels"] == all_channels:
                 data["all_sales_channels"] = True

src/tests/api/test_events.py

@@ -807,6 +807,19 @@ def test_event_update_plugins_validation(token_client, organizer, event, item, m
     )
     assert resp.status_code == 200
 
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/'.format(organizer.slug, event.slug),
+        {
+            "all_sales_channels": False,
+            "limit_sales_channels": ["web"],
+            "sales_channels": ["bar"],
+        },
+        format='json'
+    )
+    assert resp.status_code == 400
+    assert resp.content.decode() == ('{"limit_sales_channels":["If \'limit_sales_channels\' is set, the legacy '
+                                     'attribute \'sales_channels\' must not be set or set to the same list."]}')
+
 
 @pytest.mark.django_db
 def test_event_test_mode(token_client, organizer, event):