Cannot add custom language to a site when using Multilingual Blog license

Overview of the issue

When adding a new custom language to the site there is a JS error in the browser console:

Uncaught (in promise) TypeError: Cannot read properties of null (reading ‘value’)

The issue only occurs if your site is registered with a Multilingual Blog license.

Workaround

Our team is already working on the fix. Meanwhile, please follow these steps.

1. Please make sure you have a complete backup of the site.
2. Add this code to the theme’s functions.php:

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   add_action('init', function (){ $wpml_setting = get_option('WPML(setup)'); $wpml_setting['translate-everything'] = false; update_option('WPML(setup)', $wpml_setting); });

3. Visit the dashboard once then remove the code.
4. Check again it should work now.