Plugin no longer accepts doc url location

Describe the bug

When upgrading to latest version of the plugin (v3.1.1), my doc url no longer respects the getPreviewUrl inputs (slug url). Here is my code:

import { Iframe } from 'sanity-plugin-iframe-pane'

// Show the correct URL based on the current document
function getPreviewUrl(doc) {
  return doc?.slug?.current
    ? `http://localhost:3000/projects/${doc.slug.current}`
    : `${window.location.host}`
}

// Import this into the deskTool() plugin
export const defaultDocumentNode = (S, {schemaType}) => {
  // Only show preview pane on `project` schema type documents
  switch (schemaType) {
    case `project`:
      return S.document().views([
        S.view.form(),
        S.view
          .component(Iframe)
          .title('Preview')
          .options({
            url: (doc) => getPreviewUrl(doc) + '?preview=true',
          })
          .title('Preview')
      ])
    default:
      return S.document().views([S.view.form()])
  }
}

This code returns the slug null3333?preview=true. When downgrading to v2.6.1, the slug is correctly rendered: http://localhost:3000/projects/my-project?preview=true.

The current solution is to downgrade plugin versions.

Expected behavior

The correct slug/url should display within the plugin config. Perhaps my code is incorrect? Seems odd since this is the same code structure I have used for the past year or more...

sanity-io / sanity-plugin-iframe-pane

Plugin no longer accepts doc url location #88