0.20.x/Material-docs theme: showing blank page when visting /section/ even if there's an index.md in that directory

github issue: https://github.com/spf13/hugo/issues/3373

reproduce: https://themes.gohugo.io/theme/material-docs/ and click the “Getting started” link in the menu.

This works as expected in hugo 0.19.

I’m not sure if this is a bug in the theme or in hugo…

@digitalcraftsman Just from a quick look, does this have anything to do with using index.md as a single page in the demo site rather than the preferred _index.md?