To reproduce:

1. Drag a grid from the Toolbox and drop it onto the form
2. Open the designer
3. Set the RadGridView.AutoGenerateHierarchy property to true.
4. Open the smart tag and set the DataSource property. As a result the grid templates and relations will be created.
5. Try to open the Property Builder. It opens successfully.
6. Close the Property Builder and add a new templated from the smart tag.
7. If you try to open the Property Builder again you will obtain the error. Please refer o the attached gif file illustrating the behavior.

Workaround: set the AutoGenerateHierarchy to false in order to open the Property Builder.