I the preview-theme script there are some errors. (There are some LSP errors too but those are for a different repo)
You can see my cursor on the single quote and the error to the right.

Changing that to double quotes fixes the problem, as you can see in the screenshot below. However, these quotes should be interchangeable here.

There are several more errors with this file that may not be as easy to demonstrate but they all say ERROR with InspectTree.