Function called outside component initialization

🐛 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 get the error

Function called outside component initialization

when mounting a Svelvet component (Svelvet is a npm package)

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

We use CodeSandbox to demo components to users. All demos are now broken.

To Reproduce

Import and mount a Svelvet component (see sandbox below)

Link to sandbox:

This is an example of a CodeSandbox which was working as of yesterday and today is not:
https://codesandbox.io/s/custom-nodes-forked-1ml2fb?from-embed=&file=/App.svelte

Some notes:

• The error message matches a previously reported bug: https://github.com/codesandbox/codesandbox-client/issues/6293
• The previous bug was reported fixed. However, when I looked at the CodeSandbox for the previous bug (https://codesandbox.io/s/flamboyant-pond-wohvg?file=/App.svelte) it appears to have returned; clicking on the colored div gives the error: Function called outside component initialization

Your Environment

Start by reproducing the failure in the linked CodeSandbox with the Svelvet component and compare it with the previously reported issue #6293. Identify which change causes the “Function called outside component initialization” error and confirm the fix by remounting the affected demo without the error.