Started from scratch, and got rid of the default.html layout, which included nav.html from just-the-docs, which was causing this error locally:
Liquid Exception: Liquid error (/Library/Ruby/Gems/2.6.0/gems/just-the-docs-0.3.2/_includes/nav.html line 52): Cannot sort a null object. included in /_layouts/default.html
Started from scratch, and got rid of the default.html layout, which included nav.html from just-the-docs, which was causing this error locally: Liquid Exception: Liquid error (/Library/Ruby/Gems/2.6.0/gems/just-the-docs-0.3.2/_includes/nav.html line 52): Cannot sort a null object. included in /_layouts/default.html