"There are two ways to write error-free programs; only the third one works" (Alan J. Perlis)
Sitefinity Thunder enables fast and smooth creation of widget designers for existing widgets. You can take advantage of the Visual Studio plugin to speed up the development of your custom widgets.
When you try to add a designer for existing widget, using Sitefinity Thunder you receive a blank dialog, instead of listing the project's widgets to select from:
Make sure that you haven't deleted the SampleTemplate.cshtml file in the root of the SitefinityWebApp. Although this file is only concerning MVC-related projects, it is the reason why your Thunder dialog is empty. If you do not have the file, add it in the root of your web application as shown below:
Compile the project and test again.
NOTE: Valid for ASP.NET Web Forms web applications only!