Can't add a comment when BlockNote is the default WYSIWYG editor

Steps to reproduce

1. Set BlockNote as the default WYSIWYG editor.
2. Open a page in view mode, open the Comments tab and type a comment.
3. Click "Add comment".

Expected

The comment is added.

Actual

Nothing happens, and the browser console shows Uncaught TypeError: Cannot read properties of undefined (reading 'value').

Cause

The submit handler of the comment form in comments.js reads the comment from the first textarea of the form, after firing xwiki:actions:beforeSave to let the rich editor update its form fields:

document.fire('xwiki:actions:beforeSave');
if (form.down('textarea').value != "") {

This works with CKEditor because it replaces a real textarea. BlockNote doesn't use a textarea: it outputs a div host plus hidden input fields, so form.down('textarea') is undefined. The comments JavaScript needs to read the submitted value in a way that doesn't depend on the editor, e.g. from the form field named after the comment property.

Found while looking into XWIKI-25433, which would have CKEditor drop its textarea too in standalone edit mode.