Telerik.Windows.Navigation and RadWindow

I have just created a WPF application with telerik controls and i'm trying to create a RadWindow. For RadWindow the class is Telerik.Windows.Controls and i have added it. But still the xaml and xaml.cs file shows me error. When i add the Telerik.Windows.Navigation to the reference(not in the namespace)the error goes away.

The problem here is i have added a Button inside the form and just click of the Button throws me "Object reference error". Not sure why this is happening.

Xaml:
//Error here showing RadWindow does not exist in the namespace

<telerik:RadWindow x:Class="ProgressBar.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Height="100" Width="200" >
    <Grid>
        <Button Content="Click here"></Button>
    </Grid>
    </telerik:RadWindow>

Xaml.cs:

using System.Windows;
using Telerik.Windows.Controls;

namespace ProgressBar
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : RadWindow //shows me assembly missing
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

Why should i use the Navigation reference for my RadWindow. I believe it is added in the Controls namepsace itself.

Exception on Button Click after adding Navigation in reference,

System.NullReferenceException was unhandled
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=Telerik.Windows.Controls.Navigation
  StackTrace:
       at Telerik.Windows.Controls.WindowBase.OnGotKeyboardFocus(KeyboardFocusChangedEventArgs e)

FYI, Telerik version: WPF Q3 2014 WPF45