Uncaught TypeError in comment-reply.js when 'id_form' is not 'commentform'

[raamdev]

When comment_form() is passed and id_form argument other than the default of commentform, clicking the Reply link on a comment produces the following error in the JavaScript console:

Uncaught TypeError: Cannot read property 'addEventListener' of null

This results in the page refreshing and defaulting to the non-JS behavior, breaking the intended functionality of moving the comment reply form underneath the comment being replied to.

It looks like this bug was introduced in r45790, where it's assumed that commentFormElement contains a non-null reference to the comment form ID (config.commentFormId), which is set to an ID of commentform. If the ID is anything other than commentform, the Uncaught TypeError is produced.

To reproduce the bug in trunk, you can modify comment_form() in twentynineteen/inc/template-tags.php to specify a custom comment form element id attribute (anything other than the default of commentform produces the bug):

<?php
comment_form(
                array(
                        'logged_in_as' => null,
                        'title_reply'  => null,
                        'id_form' => 'commentform-' . get_the_ID()
                )
        );

[raamdev]

Checks if commentFormElement is non-null before using it

[SergeyBiryukov]

Thanks for the ticket and the patch! Moving to 5.3, as this appears to be a regression.

[SergeyBiryukov]

In 46700:

Comments: Check if comment form element exists before adding a key handler to detect the cmd/ctrl-enter key press.

Follow-up to [45790].

Props raamdev.
Fixes #48543. See #41545.

[SergeyBiryukov]

Reopening for a second committer's review before merging to the 5.3 branch.

[whyisjake]

In 46706:

Comments: Check if comment form element exists before adding a key handler to detect the cmd/ctrl-enter key press.

Brings [46700] to the 5.3 branch.

Follow-up to [45790].
Props raamdev.
Fixes #48543. See #41545.