Sorry, I am not able to replicate your issue.
Customizer is loading and working properly on my test site.

Please use the following troubleshooting steps.

Check your software version.

1 ) Check if you are using latest Hueman and Hueman Addons plugin, Update them if you are not using latest version.

Next step is to check for plugin conflict.

1) If you are using a server cache or cache plugin, please clear it’s cache content and disable caching via it’s settings.

2) Proceed to disable one plugin at a time, clear browser history and re-visit your customizer. Check if it’s working properly.

Do this until you find the plugin that’s causing this issue, please replace with an alternative plugin.

Thank you