react mdx example fails when adding second storybook config to project

[patsplat]

Describe the bug
Encountered an import error when adding a second storybook config with --story-format mdx to an existing storybook react installation.

To Reproduce
Steps to reproduce the behavior:

1. npx create-react-app my-app
2. cd my-app
3. npx sb init
4. mkdir nested-config
5. cd nested-config
6. npx sb init -s -t react --story-format mdx
7. cd ..
8. yarn storybook -c nested-config/.storybook
9. See Error: Element type is invalid

Expected behavior
Storybook to start without errors

Screenshots

System:

Environment Info:

  System:
    OS: macOS Mojave 10.14.6
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
  Binaries:
    Node: 10.16.3 - ~/.nvm/versions/node/v10.16.3/bin/node
    Yarn: 1.21.1 - ~/.yarn/bin/yarn
    npm: 6.13.4 - ~/.nvm/versions/node/v10.16.3/bin/npm
  Browsers:
    Chrome: 84.0.4147.135
    Safari: 13.1.2
  npmPackages:
    @storybook/addon-actions: ^6.0.18 => 6.0.18 
    @storybook/addon-essentials: ^6.0.18 => 6.0.18 
    @storybook/addon-links: ^6.0.18 => 6.0.18 
    @storybook/node-logger: ^6.0.18 => 6.0.18 
    @storybook/preset-create-react-app: ^3.1.4 => 3.1.4 
    @storybook/react: ^6.0.18 => 6.0.18

Additional context
This error was originally encountered in the context of a production monolith managed by a large team. The create-react-app example is provided to reproduce the error.

There is a bug fix for the import error in #12252

[shilman]

Crikey!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.0.20 containing PR #12252 that references this issue. Upgrade today to try it out!

Closing this issue. Please re-open if you think there's still more to do.