Handle null asset types.

backend/src/Squidex.Domain.Apps.Entities/Contents/GraphQL/Types/Assets/AssetsResultGraphType.cs

@@ -30,7 +30,7 @@ namespace Squidex.Domain.Apps.Entities.Contents.GraphQL.Types.Assets
             AddField(new FieldType
             {
                 Name = "items",
-                ResolvedType = assetsList,
+                ResolvedType = new NonNullGraphType(assetsList),
                 Resolver = ResolveList(x => x),
                 Description = "The assets."
             });

backend/src/Squidex.Domain.Apps.Entities/Contents/GraphQL/Types/SharedTypes.cs

@@ -46,7 +46,7 @@ namespace Squidex.Domain.Apps.Entities.Contents.GraphQL.Types
 
             assetsList = new Lazy<IGraphType>(() =>
             {
-                return new NonNullGraphType(new ListGraphType(new NonNullGraphType(Asset)));
+                return new ListGraphType(new NonNullGraphType(Asset));
             });
 
             assetsResult = new Lazy<IGraphType>(() =>