Thank you for contacting Telerik support. If you have TestFieldControl placed in a MyWidgets folder inside Sitefinity WebApp, then you have to enter "SitefinityWebApp.MyWidgets.TestFieldControl" for "Type or Virtual path of the custom widget" in Sitefinity backend when you add your custom field. Please, find screenshots I attached. Setting incorrect type of the field control might be causing the error you are experiencing.
Regards, Miroslava Telerik
Actually the label under textbox for entering your field control is confusing for those who create and use field controls for the first time. You have to enter the full type to the field. That is why we generate a clarifying comment in the field control code:
/// A simple field control used to save a string value.
/// Use the path to this class when you add the field control
/// SitefinityWebApp.<your field control>
Here is a video showing step by step how to create and use a field control: