v16: super before this access

[ChristopherBiscardi]

I think I forgot a super() in the MDXContent class from #309. Not sure why it didn't pop up before when running tests, etc.

[tujoworker]

Thanks a lot for Your work @ChristopherBiscardi so far!
Since Version 0.16.1 of @mdx-js/mdx i get this error: MDXContent cannot be invoked without 'new'

gatsby-mdx is using your work as a peer dependency import MDXRenderer from 'gatsby-mdx/mdx-renderer'

Hope this is not too much as a pain to fix ☺️

[ChristopherBiscardi]

@tujoworker Check out ChristopherBiscardi/gatsby-mdx#192

I have it working locally but need to find a moment to push it up, merge and release.

[ChristopherBiscardi]

(just to make it clear for any other maintainers, this is a gatsby-mdx runtime issue and not a mdx-js/mdx issue)