docs(gantt): Fix breaking ObservableCollection example

Author: dimodi

components/gantt/refresh-data.md

@@ -239,14 +239,11 @@ In this article:
     
     private GanttView SelectedView { get; set; } = GanttView.Week;
 
-
-    void AddRootTask()
+    private void AddRootTask()
     {
-        var i = GanttData.Last().Id + 1;
-
         GanttData.Insert(0,new FlatModel()
         {
-            Id = i,
+            Id = ++LastId,
             ParentId = null,
             Title = "new task",
             PercentComplete = 0,
@@ -522,4 +519,4 @@ In this article:
 
   * [ObservableCollection]({%slug common-features-observable-data%})
   * [INotifyCollectionChanged Interface](https://docs.microsoft.com/en-us/dotnet/api/system.collections.specialized.inotifycollectionchanged?view=netframework-4.8)
-  * [Live Demos](https://demos.telerik.com/blazor-ui/)
+  * [Live Demos](https://demos.telerik.com/blazor-ui/)