"Cannot read property 'index' of null" in mdx-deck 1.6.7 "Notes" on build

[trevordmiller]

When using Notes like

import { Notes } from 'mdx-deck'

<Notes>
  Only visible in presenter mode
</Notes>

When I run mdx-deck build I get TypeError: Cannot read property 'index' of null originating from node_modules/mdx-deck/dist/Notes.js:44:24:

in mdx-deck 1.6.7

[trevordmiller]

This works though:

```notes
These notes are only visible in presenter mode
```

[pomber]

There's a similar problem with Appear since v1.6.6:

[err] TypeError: Cannot read property 'step' of null
    at Appear.render (node_modules/mdx-deck/dist/Appear.js:81:30)
    ...

BTW both Notes and Appear work when using the --no-html flag.

[jxnblk]

Thanks! I'm able to reproduce, I imagine it has something to do with webpack-node-externals not being set up correctly, but I'm looking into it