docs(treeView): flat binding requires at least one null parentId

Author: marin-bratanov

components/treeview/data-binding/flat-data.md

@@ -16,7 +16,7 @@ This article explains how to bind the TreeView for Blazor to flat data.
 
 Flat data means that the entire collection of treeview items is available at one level, for example `List<MyTreeItemModel>`.
 
-The parent-child relationships are created through internal data in the model - the `ParentId` field which points to the `Id` of the item that will contain the current item. The root level has `null` for `ParentId`.
+The parent-child relationships are created through internal data in the model - the `ParentId` field which points to the `Id` of the item that will contain the current item. The root level has `null` for `ParentId`. There must be at least one node with a `null` value so that the TreeView renders anything.
 
 You must also provide the correct value for the `HasChildren` field - for items that have children, you must set it to `true` so that the expand arrow is rendered.