Chat archive export button does nothing and throws TypeError in revealModal

[mysubcult]

Description

When opening the chat archive page in the admin panel and clicking the Export button, nothing happens. No export file is downloaded and no modal is shown. The browser console shows a JavaScript error.

Steps to reproduce

1. Log in to the Live Helper Chat admin panel.
2. Open a chat archive page, for example:
   /index.php/site_admin/chatarchive/listarchivechats/2
3. Click the Export button.

Actual behavior

Nothing happens after clicking Export.

The browser console reports the following error:

Uncaught TypeError: 'querySelector' called on an object that does not implement interface Element.
    findOne ... line 2 > injectedScript:4
    _showElement ... line 2 > injectedScript:4
    show ... line 2 > injectedScript:4
    revealModal /design/defaulttheme/js/lh/dist/740-632a28333d24f7ab53a2.js:1
    c /cache/compiledtemplates/f1067c57b3c5b5489021f3a9538efa03.js:2
    fireWith /cache/compiledtemplates/f1067c57b3c5b5489021f3a9538efa03.js:2
    l /cache/compiledtemplates/f1067c57b3c5b5489021f3a9538efa03.js:2
    o /cache/compiledtemplates/f1067c57b3c5b5489021f3a9538efa03.js:2