Changes the CreateTreeNode to ensure that a parentId is always passed in - now all tree node models have a parentId property based on the server side data. Changes the JS tree node model to not have a 'parent' property and instead changes this to a parent() method - this allows us to serialize the tree model because otherwise it has cyclical references.

src/Umbraco.Web/Models/Trees/SectionRootNode.cs

@@ -1,4 +1,5 @@
 using System.Runtime.Serialization;
+using Umbraco.Core;
 
 namespace Umbraco.Web.Models.Trees
 {
@@ -33,7 +34,7 @@ namespace Umbraco.Web.Models.Trees
         }
 
         private SectionRootNode(string nodeId, string getChildNodesUrl, string menuUrl)
-            : base(nodeId, getChildNodesUrl, menuUrl)
+            : base(nodeId, null, getChildNodesUrl, menuUrl)
         {
             //default to false
             IsContainer = false;