I found a discussion from 2014 that indicates this was a problem in the past but that it has since been fixed. But, in 2024, I seem to have this problem again.
The only thing I can do is create a new module, either using the File->Open Spec... menu options, or the Spec Explorer.
I'm on Ventura 13.5.2. My TLA+ Toolbox is `Version 1.7.1 of 31 December 2020`.