Steps to recreate:
1. Create an assignment
2. Add the comments block to the assignment page
3. Configure the block:
Set 'Display on pages' to 'Any assignment module page'
4. Navigate to submission grading page: mod/assign/view.php?id=#&action=grader&userid=#

Replicated on https://school.moodledemo.net/my/courses.php. The bug does not occur when using classic theme but does occur using Boost.

Error:
comment.js:22 Uncaught TypeError: Cannot read properties of null (reading 'getStyle')
at CommentHelper.view (comment.js:22:1001)
at CommentHelper.initializer (comment.js:1:556)
at CommentHelper._initHierarchy (yui_combo.php?rollup/3.17.2/yui-moodlesimple.js:28941:33)
at CommentHelper._baseInit (yui_combo.php?rollup/3.17.2/yui-moodlesimple.js:28508:18)
at CommentHelper.init (yui_combo.php?rollup/3.17.2/yui-moodlesimple.js:29628:22)
at CommentHelper._initBase (yui_combo.php?rollup/3.17.2/yui-moodlesimple.js:28472:23)
at CommentHelper.BaseCore (yui_combo.php?rollup/3.17.2/yui-moodlesimple.js:28292:18)
at CommentHelper.Base (yui_combo.php?rollup/3.17.2/yui-moodlesimple.js:29842:18)
at new CommentHelper (comment.js:1:112)
at Object.init (comment.js:30:440)