I am trying to create a custom tree in umbraco 8.1.3 by following this tutorial https://our.umbraco.com/documentation/Extending/Section-Trees/trees. However, I get this error 'Constants' is inaccessible due to its protection level on this line: id == Constants.System.Root.ToInvariantString()

Here are the namespaces Included in the class: using System; using System.Collections.Generic; using System.Linq; using System.Net.Http.Formatting; using System.Web; using System.Web.Http.ModelBinding; using Umbraco.Web.Actions; using Umbraco.Web.Models.Trees; using Umbraco.Web.Mvc; using Umbraco.Web.Trees; using Umbraco.Web.WebApi.Filters;