`TypeError: e is undefined` in page preview

Describe the bug When I edit a certain page on my site based upon netlify-templates/gatsby-starter-netlify-cms, I get the following error in the preview pane:

TypeError: e is undefined

To Reproduce

1. Go to https://quizzical-tesla-b8c593.netlify.com/admin (Built from https://gitlab.com/aidalgol/netlify-cms-gatsby-preview-bug)
2. Create a Netlify Identity user through Netlify CMS
3. Log in to the Netlify CMS admin panel
4. Edit the page named “Landing Page”
5. The preview pane on the right-hand side will show the error described earlier.

Expected behavior The page’s component to render normally.

Screenshots

Applicable Versions:

• Netlify CMS version: 2.9.7
• Git provider: GitLab
• OS: Debian 10
• Browser version Firefox 60.8.0esr

CMS configuration https://gitlab.com/aidalgol/netlify-cms-gatsby-preview-bug/blob/master/static/admin/config.yml

[erquhart]

It’s better running locally, but still goes to the built es modules rather than source. Very readable, not minified.

_{Sent with GitHawk}

[aidalgol]

Is there anything I can do in future to get a more useful stacktrace, such as running a local dev build of Netlify CMS?