Trouble with ToolWindows in a ControlTemplate

<Window x:Class="DockingSample.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
    xmlns:shared="http://schemas.actiprosoftware.com/winfx/xaml/shared"
    xmlns:local="clr-namespace:DockingSample"
    Title="Window1" Height="600" Width="600">
    
    <Window.Resources>
        <ControlTemplate x:Key="testTemplate" TargetType="{x:Type local:CustomControl}">
            <docking:DockSite x:Name="dockSite">
                <docking:SplitContainer>
                    <docking:ToolWindowContainer x:Name="toolWindowContainer">
                        <docking:ToolWindow Title="Toolbar" x:Name="toolWindow">
                            <TextBlock x:Name="PART_Toolbar" />
                        </docking:ToolWindow>
                    </docking:ToolWindowContainer>
                    <docking:Workspace>
                        <TextBlock x:Name="PART_Content" />
                    </docking:Workspace>
                </docking:SplitContainer>
            </docking:DockSite>
        </ControlTemplate>
    </Window.Resources>
    
    <local:CustomControl Template="{DynamicResource testTemplate}" />

</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Controls;

namespace DockingSample
{
    public class CustomControl : Control
    {
        private TextBlock _toolbarText;
        private TextBlock _contentText;


        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            _toolbarText = GetTemplateChild("PART_Toolbar") as TextBlock;
            _toolbarText.Text = "something about tools...";

            _contentText = GetTemplateChild("PART_Content") as TextBlock;
            _contentText.Text = "Here is a bunch of content for the main workspace";
        }
    }
}