Error "Could not find entry file: /index.html" on `master` branch

🐛 bug report

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project
  adheres to.
• I have searched the issue tracker for an issue that matches the one I want
  to file, without success.

Description of the problem

I have a GitHub sandbox that gives me the following error:

Error
Could not find entry file: /index.html. You can specify one in package.json by defining a `main` property.

This is odd, because the problem only happens now that the sandbox has been merged on master. The sandbox generated from the working branch (now deleted was working fine).

Also, the sandbox generated from the commit hash on master works as well, even though it's exactly the same.

How has this issue affected you? What are you trying to accomplish?

We need this sandbox for demo purposes, so it's currently blocking a release.

To Reproduce

• Go to the sandbox from master, notice the error.
• Go to the the sandbox from d58c4b3, notice it works despite being the same code.

Link to sandbox: link

Your Environment

Start by reproducing the error in the linked sandbox on the master branch and compare it with the working sandbox at commit d58c4b3. Trace how the repository path is resolved and how the entry file is selected. Done means the master-branch sandbox loads successfully for the demo without the missing /index.html error.