How to add or remove nodes to unbound mode TreeView in Xamarin.Forms?

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:syncfusion="clr-namespace:Syncfusion.XForms.TreeView;assembly=Syncfusion.SfTreeView.XForms"
             xmlns:treeviewengine="clr-namespace:Syncfusion.TreeView.Engine;assembly=Syncfusion.SfTreeView.XForms"
             xmlns:behavior="clr-namespace:TreeViewXamarin.Helper"
             x:Class="TreeViewXamarin.MainPage" Padding="{OnPlatform iOS='0,40,0,0'}">
    <ContentPage.Behaviors>
        <behavior:Behavior/>
    </ContentPage.Behaviors>
    <ContentPage.Content>
        <StackLayout>
            <Grid HeightRequest="50" >
                <Button x:Name="addButton" Text="Add Node" />
                <Button x:Name="deleteButton" Text="Delete Node" Grid.Column="1"/>
            </Grid>
            <syncfusion:SfTreeView x:Name="treeView" NotificationSubscriptionMode="CollectionChange">
                <syncfusion:SfTreeView.Nodes>
                    <treeviewengine:TreeViewNode x:Name="australia" Content="Australia" >
                        <treeviewengine:TreeViewNode.ChildNodes>
                            <treeviewengine:TreeViewNode Content="New South Wales">
                                <treeviewengine:TreeViewNode.ChildNodes>
                                    <treeviewengine:TreeViewNode Content="Sydney"/>
                                    <treeviewengine:TreeViewNode Content="Canberra"/>
                                    <treeviewengine:TreeViewNode Content="Newcastle–Maitland"/>
                                </treeviewengine:TreeViewNode.ChildNodes>
                            </treeviewengine:TreeViewNode>
                           ...
                </syncfusion:SfTreeView.Nodes>
            </syncfusion:SfTreeView>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

namespace TreeViewXamarin.Helper
{
    public class Behavior : Behavior<ContentPage>
    {
        SfTreeView TreeView;
        Button AddButton;
        Button DeleteButton;
 
        protected override void OnAttachedTo(ContentPage bindable)
        {
            TreeView = bindable.FindByName<SfTreeView>("treeView");
            AddButton = bindable.FindByName<Button>("addButton");
            DeleteButton = bindable.FindByName<Button>("deleteButton");
 
            AddButton.Clicked += AddButton_Clicked;
            DeleteButton.Clicked += DeleteButton_Clicked;
 
            base.OnAttachedTo(bindable);
        }
 
        private void AddButton_Clicked(object sender, EventArgs e)
        {
            var india = new TreeViewNode() { Content = "India" };
            var delhi = new TreeViewNode() { Content = "Delhi" };
            var newDelhi = new TreeViewNode() { Content = "New Delhi" };
            delhi.ChildNodes.Add(newDelhi);
            var tamilNadu = new TreeViewNode() { Content = "Tamil Nadu" };
            var chennai = new TreeViewNode() { Content = "Chennai" };
            india.ChildNodes.Add(delhi);
            india.ChildNodes.Add(tamilNadu);
            tamilNadu.ChildNodes.Add(chennai);
 
            TreeView.Nodes.Add(india);
        }
 
        private void DeleteButton_Clicked(object sender, EventArgs e)
        {
            TreeView.Nodes.RemoveAt(1);
        }
    }
}