Steps to reproduce the issue

1. Enable Inspector in Chrome
2. Go to the Control Panel in Joomla
3. Wait for the popup notifications to show
4. Check the Console tab of the Inspector
5. The error is shown there

Expected result

No errors

Actual result

Error is shown:
Uncaught TypeError: Cannot read property 'style' of null
at HTMLDocument. (flying-focus.min.js?7a338ea4a2cd5df7edb93e870b062de1:1)
at HTMLElement.connectedCallback (joomla-alert.min.js?7a338ea4a2cd5df7edb93e870b062de1:1)
at Object.e.renderMessages (core.min.js?7a338ea4a2cd5df7edb93e870b062de1:1)
at onSuccess (extensionupdatecheck.js?7a338ea4a2cd5df7edb93e870b062de1:47)
at XMLHttpRequest.o.onreadystatechange (core.min.js?7a338ea4a2cd5df7edb93e870b062de1:1)
flying-focus.min.js?7a338ea4a2cd5df7edb93e870b062de1:1

The error is a little different in FireFox:
TypeError: l is null[Learn More] flying-focus.min.js:1:2227
http://joomla.dev/media/vendor/flying-focus-a11y/js/flying-focus.min.js:1:2227
connectedCallback http://joomla.dev/media/system/webcomponents/joomla-alert.min.js:1:4749
r.prototype.connectedCallback http://joomla.dev/media/system/js/polyfills/webcomponents/webcomponents-ce.min.js:1:4754
f http://joomla.dev/media/system/js/polyfills/webcomponents/webcomponents-ce.min.js:1:1326
http://joomla.dev/media/system/js/polyfills/webcomponents/webcomponents-ce.min.js:1:10423
e.renderMessages http://joomla.dev/media/system/js/core.min.js:1:3348
onSuccess http://joomla.dev/media/plg_quickicon_extensionupdate/js/extensionupdatecheck.js:47:9
e.request/o.onreadystatechange http://joomla.dev/media/system/js/core.min.js:1:7584

System information (as much as possible)

Joomla 4.0 Revision: ab82c24
Chrome / FireFox
MacOS 10.12.6 (Sierra)
PHP 7.1.6

@dgt41 Something for you?