@mdx-js/rollup plugin doesn't work in development mode

Discussed in #925

^{Originally posted by offsounder April 30, 2022}
Hello,

I'm trying to use the @mdx-js/rollup plugin with wmr, however it errors out when running in development mode with the following message: Cannot read properties of undefined (reading '0').

When building with wmr --prerender, the md is correctly rendered to html.

Any ideas on what is going on?

map.sourcesContent might not exist so this throws:

wmr/packages/wmr/src/lib/sourcemap.js

Line 112 in 1d30445

    
           sourcesCache.set(source, { index: sourcesCache.size, content: map.sourcesContent[j] || null });