First, as it is an Elementor issue you will get specific support from their support forum
https://wordpress.org/support/plugin/elementor/
Secondly, a production site should be set to not display warnings and notices.
Notices and Warnings are to help developers clean up code issues e.g. the Notice might be that a function is deprecated in a future release. Or a Warning may be that a variable has not been set before being used.
In general – on a production site – you should have Notices and Warning set not to display.
This is easy to do, you can edit your wp-config.php file and set
ini_set('display_errors','Off');
ini_set('error_reporting', E_ALL );
define('WP_DEBUG', false);
define('WP_DEBUG_DISPLAY', false);
Or if that is too technical – just raise a support ticket to your host to ask them to suppress PHP Notices and Warnings.
It might be triggered by either an Elementor addon (not the Elementor plugin, but companion plugins that add new components etc.) or an Elementor-ready theme. I’d recommend deactivating Elementor addons one by one until you no longer get the warning, and if you keep getting the warning, switch to another theme. Doing this will likely help you find the culprit, so you can ask an update from whoever’s responsible.
