Error in SampleProject - Helix Toolkit

When I am trying one of the example name - SimpleDemo in Helix Toolkit I am getting Invalid Markup. The code is this: It is exactly the same in the helix toolkit project.

Code:

MainWindow.xaml: 

<Window x:Class="SimpleDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:HelixToolkit="clr-namespace:HelixToolkit.Wpf;assembly=HelixToolkit.Wpf"
        xmlns:local="clr-namespace:SimpleDemo"
        Title="SimpleDemo" Height="480" Width="640">
    <Window.DataContext>
        <local:MainViewModel/>
    </Window.DataContext>
    <Grid>
        <!-- The HelixViewport3D supports camera manipulation, and can be used just like the Viewport3D -->
        <HelixToolkit:HelixViewport3D ZoomExtentsWhenLoaded="True">

            <!-- Remember to add light to the scene -->
            <HelixToolkit:SunLight/>
            
            <!-- The content of this visual is defined in MainViewModel.cs -->
            <ModelVisual3D Content="{Binding Model}"/>

            <!-- You can also add elements here in the xaml -->
            <HelixToolkit:GridLinesVisual3D Width="8" Length="8" MinorDistance="1" MajorDistance="1" Thickness="0.01"/>

        </HelixToolkit:HelixViewport3D>
    </Grid>
</Window>

MainWindow.cs

// --------------------------------------------------------------------------------------------------------------------
// <copyright file="MainViewModel.cs" company="Helix Toolkit">
//   Copyright (c) 2014 Helix Toolkit contributors
// </copyright>
// <summary>
//   Provides a ViewModel for the Main window.
// </summary>
// --------------------------------------------------------------------------------------------------------------------

namespace SimpleDemo
{
    using System.Windows.Media;
    using System.Windows.Media.Media3D;

    using HelixToolkit.Wpf;

    /// <summary>
    /// Provides a ViewModel for the Main window.
    /// </summary>
    public class MainViewModel
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="MainViewModel"/> class.
        /// </summary>
        public MainViewModel()
        {
            // Create a model group
            var modelGroup = new Model3DGroup();

            // Create a mesh builder and add a box to it
            var meshBuilder = new MeshBuilder(false, false);
            meshBuilder.AddBox(new Point3D(0, 0, 1), 1, 2, 0.5);
            meshBuilder.AddBox(new Rect3D(0, 0, 1.2, 0.5, 1, 0.4));

            // Create a mesh from the builder (and freeze it)
            var mesh = meshBuilder.ToMesh(true);

            // Create some materials
            var greenMaterial = MaterialHelper.CreateMaterial(Colors.Green);
            var redMaterial = MaterialHelper.CreateMaterial(Colors.Red);
            var blueMaterial = MaterialHelper.CreateMaterial(Colors.Blue);
            var insideMaterial = MaterialHelper.CreateMaterial(Colors.Yellow);

            // Add 3 models to the group (using the same mesh, that's why we had to freeze it)
            modelGroup.Children.Add(new GeometryModel3D { Geometry = mesh, Material = greenMaterial, BackMaterial = insideMaterial });
            modelGroup.Children.Add(new GeometryModel3D { Geometry = mesh, Transform = new TranslateTransform3D(-2, 0, 0), Material = redMaterial, BackMaterial = insideMaterial });
            modelGroup.Children.Add(new GeometryModel3D { Geometry = mesh, Transform = new TranslateTransform3D(2, 0, 0), Material = blueMaterial, BackMaterial = insideMaterial });

            // Set the property, which will be bound to the Content property of the ModelVisual3D (see MainWindow.xaml)
            this.Model = modelGroup;
        }

        /// <summary>
        /// Gets or sets the model.
        /// </summary>
        /// <value>The model.</value>
        public Model3D Model { get; set; }
    }
}

Can someone help me in getting through this?