Creating dynamic module selectors
While creating a widget designer, if you select a GUID property you must choose the type of the interface which you want to use in your designer. If you select DynamicContentSelector, you will see a field where you can select the content type of your dynamic module items.
To check what your custom module item’s type is, go to Administration » Module Builder. Open your module by clicking it. In the right, click Code reference for <Module Name>. Select the first code example and you will see in the example the following code:
Copy the type between the quotes and paste it in the designer, as shown in the following screenshot: