From what I'm understanding, it seems like the user is using the legacy editor for Markdown[1], which isn't supported by the plugin

[1]

Should there be a check what kind of editor is enabled to see if the extension should work or not?

I think there's a list of unsupported plugins somewhere in the code

Note: The list of unsupported plugins is for the new Markdown editor, not the legacy editor. We do state, however, that using the legacy editor will break certain plugins:

One possible solution here could be to prompt the user to disable the legacy editor if there's a fatal error.

@pedr, could you do this then? Detect if it's CodeMirror 5 and if it is, display a message