Changed needed as GetApplicationTrees returns an IEnumerable of SectionRootNodes now

src/Umbraco.Web/Editors/SectionController.cs

@@ -50,9 +50,13 @@ namespace Umbraco.Web.Editors
                 {
                     //get the first tree in the section and get it's root node route path
                     var sectionTrees = appTreeController.GetApplicationTrees(section.Alias, null, null).Result;
-                    section.RoutePath = sectionTrees.IsContainer == false || sectionTrees.Children.Count == 0
-                        ? sectionTrees.RoutePath
-                        : sectionTrees.Children[0].RoutePath;
+
+                    //Root node trees are now in collection
+                    var firstTree = sectionTrees.FirstOrDefault();
+
+                    section.RoutePath = firstTree.IsContainer == false || firstTree.Children.Count == 0
+                        ? firstTree.RoutePath
+                        : firstTree.Children[0].RoutePath;
                 }
             }