When marking a student's submission using assignfeedback_annotatepdf, it isn't possible to add a quicklist comment to an empty comment post-it.

Feedback from our academic staff is "it used to work", so I've tested the behaviour on 3 Moodle sites I've got lying around. This feature worked in Moodle 3.7.3+, but is broken in Moodle 3.8.4+ and Moodle 3.8.5+. Tested on vanilla sites with zero third-party plugins.

Steps to Reproduce

Setup

• Create a new course, and assign a user as a student.
• Create an assignment on the course, and enable the PDF Annotation feedback plugin.
• Log in as the student and submit a file to the assignment.
• Log in as a staff member or admin, and grade the student's assignment.
• Add a comment, type some text, and save that comment to the quicklist using the menu in the corner of the comment.
• Add another comment to the document. Leave it blank, click the menu in the corner of the comment, and choose the existing comment from the quicklist.

Expected Behaviour

• As you mouse over the menu, the new empty comment stays open.
• When you click the existing comment in the menu, the text appears in the new comment.

Observed Behaviour

• As you mouse over the menu, the new empty comment disappears completely.
• Clicking on the existing comment in the menu results in a Javascript error in the browser's console. In Safari: TypeError: undefined is not an object (evaluating 'this.drawable.nodes[0].one'). In Firefox: TypeError: this.drawable.nodes[0] is undefined.

Workaround

• Typing something into the new comment box before opening the menu and choosing a quicklist comment, even just 'a', keeps the comment open and stops the error.