Id fix.

backend/src/Squidex.Domain.Apps.Entities.MongoDb/Assets/MongoAssetRepository.cs

@@ -150,7 +150,7 @@ namespace Squidex.Domain.Apps.Entities.MongoDb.Assets
             using (Profiler.TraceMethod<MongoAssetRepository>())
             {
                 var assetEntities =
-                    await Collection.Find(x => x.IndexedAppId == appId && !x.IsDeleted && x.ParentId == parentId).Only(x => x.DocumentId)
+                    await Collection.Find(x => x.IndexedAppId == appId && !x.IsDeleted && x.ParentId == parentId).Only(x => x.Id)
                         .ToListAsync();
 
                 return assetEntities.Select(x => DomainId.Create(x[Fields.AssetId].AsString)).ToList();