[Bug]: "ExternalDocs from @storybook/blocks fails with [Error reading preview.js]"

[zinchenkoivan]

Describe the bug

When using ExternalDocs from @storybook/blocks, it throws different errors depending on the environment:

In Next.js → Fails with:
Error reading preview.js

In a React + Vite setup → Fails with:
document is not defined

Current Setup:

• Storybook version: 8.6.0
• Next.js version: 15
• React version: ^19.0.0

Previously Working Setup:

• Storybook version: 7.6.3
• Next.js version: 14
• React version: 18

Reproduction link

https://codesandbox.io/p/github/zinchenkoivan/repro-bug-next/main?embed=1

Reproduction steps

1. Run the project.
2. Navigate to the file: src/app/page.tsx.

System

System:
    OS: macOS 15.3.1
    CPU: (10) arm64 Apple M1 Max
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.10.0 - ~/.nvm/versions/node/v20.10.0/bin/node
    Yarn: 1.22.21 - /opt/homebrew/bin/yarn
    npm: 10.2.3 - ~/.nvm/versions/node/v20.10.0/bin/npm <----- active
    pnpm: 9.1.2 - /opt/homebrew/bin/pnpm
  Browsers:
    Chrome: 133.0.6943.142
    Safari: 18.3
  npmPackages:
    @storybook/addon-essentials: ^8.6.2 => 8.6.2 
    @storybook/addon-onboarding: ^8.6.2 => 8.6.2 
    @storybook/blocks: ^8.6.2 => 8.6.2 
    @storybook/experimental-addon-test: ^8.6.2 => 8.6.2 
    @storybook/experimental-nextjs-vite: ^8.6.2 => 8.6.2 
    @storybook/react: ^8.6.2 => 8.6.2 
    @storybook/test: ^8.6.2 => 8.6.2 
    eslint-plugin-storybook: ^0.11.3 => 0.11.3 
    storybook: ^8.6.2 => 8.6.2

Additional context

Repository for reproduction (if needed): GitHub Repo.

[shilman]

First I want to point out that you're using an intentionally undocumented feature. Nevertheless, let's see what we can do. I suspect what's going on here is that there are two versions of React. @ndelangen can you comment here?