The problem of a novice WPF designer

I am writing my first WPF application and I am trying to implement a fade out animation when a form closes. I came across this question Wpf window fade out on close which shows how to make a fade out animation, but I can't get it to work. I have this in my XAML:

<Window.Resources>
    <Storyboard Name="FadeOutStoryboard" x:Key="FadeOutStoryboard" Completed="FadeOutStoryboard_Completed">
        <DoubleAnimation Storyboard.TargetProperty="Window.Opacity" From="1" To="0" Duration="0:0:2" FillBehavior="HoldEnd" />
    </Storyboard>
</Window.Resources>

      

        
        
        
      

    

And I have this event handler:

    private bool doneFade;
    private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
    {
        if (!doneFade)
        {
            e.Cancel = true;
            Storyboard sb = (Storyboard)this.FindResource("FadeOutStoryboard");
            sb.Begin();
        }
    }

      

        
        
        
      

    

But when the method is called sb.Begin()

, I get this exception:

System.InvalidOperationException: No target was specified for 'System.Windows.Media.Animation.DoubleAnimation'.

      

        
        
        
      

    

As said, this is my first attempt at WPF, so I rather agree on what I need to do to add a fade when the form is closed.