Commenting again to add that it looks like the reason I wasn't seeing anything in the thumbnail may have been because I kept the "Desktop & Screen Saver" window

Commenting again to add that it looks like the reason I wasn't seeing anything in the thumbnail may have been because I kept the "Desktop & Screen Saver" window with the thumbnail open while working on it and installing new versions of it. So the missing thumbnail may have been an artifact of the thumbnail showing an earlier version of the code or something, even though it was correctly showing the latest version when running the full screen version. It may also have been related to the Info.plist data of the project. Not sure. But if anyone else experiences the same thing, try to navigate back to the main "System Preferences" view and into the "Desktop & Screen Saver" view again and the thumbnail might update. But if you still run into an all-black thumbnail preview then more investigation will be necessary.