Apologies if this is a duplicate - haven't found anything yet.

It appears that in order for a particular user to comment on a question in the question bank they need a variety of permissions:

• moodle/question:commentall
• moodle/question:commentmine
• AND
• moodle/comment:post 

It was very unclear that we needed the moodle/comment:post permission and then more surprising to discover it must be given at a System context rather than course or similar. 

If this permission is not granted then rather than a nice failure message about permissions or similar you get:

"undefined is not an object (evaluating 'document.querySelectorAll("div.comment-area a")[0].style')"

Steps to replicate:

• Have a user role with both moodle/question:commentall and moodle/question:commentmine allowed, but no moodle/comment:post
• Go to a question bank within a course and attempt to leave a comment
• Get error when comment modal opens.