notes - why isn't this building?

src/Umbraco.Web.Common/Extensions/ApplicationBuilderExtensions.cs

@@ -28,6 +28,9 @@ namespace Umbraco.Extensions
         /// </summary>
         public static IApplicationBuilder UseUmbraco(this IApplicationBuilder app)
         {
+            // TODO: Should we do some checks like this to verify that the corresponding "Add" methods have been called for the
+            // corresponding "Use" methods?
+            // https://github.com/dotnet/aspnetcore/blob/b795ac3546eb3e2f47a01a64feb3020794ca33bb/src/Mvc/Mvc.Core/src/Builder/MvcApplicationBuilderExtensions.cs#L132
             if (app == null)
             {
                 throw new ArgumentNullException(nameof(app));