gatsby: "registerPreviewStyle" error with gatsby-plugin-netlify-cms

Description

The latest gatsby-plugin-netlify-cms doesn’t seem to work with the latest netlify-cms.

Steps to reproduce

Install gatsby-plugin-netlify-cms@3.0.0-rc.1 and netlify-cms@2.8.0 and load up the /admin/ page.

Expected result

User should see the admin page with no console errors

Actual result

A blank screen with the following error is seen when loading up the Netlify CMS admin page:

cms.js:10 Uncaught TypeError: Cannot read property 'registerPreviewStyle' of undefined
    at Object.<anonymous> (cms.js:10)
    at __webpack_require__ (bootstrap:19)
    at Object.<anonymous> (bootstrap:83)
    at __webpack_require__ (bootstrap:19)
    at bootstrap:83
    at bootstrap:83

Environment

  System:
    OS: macOS High Sierra 10.13.6
    CPU: (8) x64 Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 11.6.0 - /var/folders/qv/2cfwn7m13jb4rpxhm9trf1yczc9jvk/T/yarn--1553291763090-0.49216429764230507/node
    Yarn: 1.13.0 - /var/folders/qv/2cfwn7m13jb4rpxhm9trf1yczc9jvk/T/yarn--1553291763090-0.49216429764230507/yarn
    npm: 6.5.0-next.0 - ~/.nvm/versions/node/v11.6.0/bin/npm
  Languages:
    Python: 2.7.15 - /usr/local/bin/python
  Browsers:
    Chrome: 73.0.3683.86
    Firefox: 55.0.3
    Safari: 11.1.2
  npmPackages:
    gatsby: ^2.2.0 => 2.2.0 
    gatsby-image: ^2.0.0-rc.1 => 2.0.34 
    gatsby-link: ^2.0.0-rc.2 => 2.0.16 
    gatsby-plugin-canonical-urls: ^2.0.0-rc.1 => 2.0.12 
    gatsby-plugin-emotion: ^4.0.1 => 4.0.6 
    gatsby-plugin-esca-css: ^1.0.3 => 1.0.3 
    gatsby-plugin-favicon: ^3.1.4 => 3.1.5 
    gatsby-plugin-feed: ^2.0.8 => 2.0.15 
    gatsby-plugin-google-analytics: ^2.0.0-rc.1 => 2.0.17 
    gatsby-plugin-google-fonts: ^0.0.4 => 0.0.4 
    gatsby-plugin-netlify: ^2.0.0-rc.2 => 2.0.13 
    gatsby-plugin-netlify-cms: ^3.0.0-rc.1 => 3.0.16 
    gatsby-plugin-netlify-cms-paths: ^1.0.1 => 1.2.1 
    gatsby-plugin-polyfill-io: ^1.0.5 => 1.1.0 
    gatsby-plugin-prefetch-google-fonts: ^1.2.0 => 1.4.0 
    gatsby-plugin-react-helmet: ^3.0.10 => 3.0.10 
    gatsby-plugin-remove-trailing-slashes: ^2.0.0-rc.1 => 2.0.10 
    gatsby-plugin-robots-txt: ^1.3.0 => 1.4.0 
    gatsby-plugin-sharp: ^2.0.0-rc.3 => 2.0.29 
    gatsby-plugin-sitemap: ^2.0.10 => 2.0.10 
    gatsby-plugin-webpack-size: ^0.0.3 => 0.0.3 
    gatsby-remark-copy-linked-files: ^2.0.0-rc.1 => 2.0.11 
    gatsby-remark-external-links: ^0.0.4 => 0.0.4 
    gatsby-remark-images: ^3.0.1 => 3.0.10 
    gatsby-remark-smartypants: ^2.0.0-rc.1 => 2.0.9 
    gatsby-source-filesystem: ^2.0.1-rc.1 => 2.0.27 
    gatsby-transformer-remark: ^2.3.6 => 2.3.6 
    gatsby-transformer-sharp: ^2.1.1-rc.2 => 2.1.17