Visual Studio XAML设计窗口错误：未将对象引用设置为对象的实例

I have a UserControl that I am trying to use in a WPF application. It is a dll that takes a text string, converts it to a shape and renders the shape. Sort of a custom textblock that can be stroked, filled, etc as a shape. I can use it in my project just fine, for a while. Then, it stops showing the control in the design window and I get the Object reference error. If I build and run the project, the control works. Only the design window seems to have a problem with it. This is the code for the control:I have a UserControl that I am trying to use in