Error: "Value cannot be null and Parameter name: window"

A Window1.xaml file reads as follows:

<Window x:Class="WpfApplication1.Window1"
    xmlns="as usual"
    xmlns:x="as usual"
    xmlns:rtf="clr-namespace:RTFEditor;assembly=RTFEditor"
    Title="Window1" Height="300" Width="300">
 <DockPanel>
  <rtf:RTFBox></rtf:RTFBox>
 </DockPanel>

The XAML compiler complains that  <rtf:RTFBox></rtf:RTFBox>"Value cannot be null and Parameter name: window" and throws an error.   What's wrong with the rtf as defined above?

The RTFBox class for UserControl purpose is using the namespace RTFEditor which co-exists with the Application namespace: WpfApplication1. Then RTFBox () is constructed as :

 public RTFBox()
        {
            this.InitializeComponent();            
        }

Even with this error revealed the application still runs but my question is: Is there a way to rectify the problem, if identified? I need help. Thanks for your help in advance.

Mark