When using a resource selector in a Smartform, you may recieve the following error when adding/editing the selected content or folder item

Object reference not set to an instance of an object.

The root content folder doesn't have a default template selected.

In order to fix the issue, please follow the steps below

1. Navigate to the root folder in the workarea ("Folders")
2. Edit the folder properties
3. In the template configuration, you may notice either no template or no template marked as the "default template." If you see either of these, add a template and mark one of the templates as the default
4. Test the resource selector again
5. If the above steps do not resolve the issue, please contact support.