I started using RadTreeView, so I added XAML reference and tree to page, and corresponding collection in viewmodel.
After add Item to collection I got exception as below:
05-22 14:54:25.831 E/mono-rt ( 4107): [ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object.
at Telerik.XamarinForms.DataControls.RadTreeView.UpdateParentItemOnAddRemove
at Telerik.XamarinForms.DataControlsRenderer.Android.TreeViewRenderer.LayoutItemAdded
at Telerik.XamarinForms.DataControls.TreeView.TreeViewLayout.RaiseLayoutItemAdded
at Telerik.XamarinForms.DataControls.TreeView.TreeViewLayout.AddItem
at Telerik.XamarinForms.DataControls.TreeView.TreeViewLayout.OnCollectionChanged
at System.Collections.ObjectModel.ObservableCollection`1[T].OnCollectionChanged