Browser alert on every post saving

[PokatilovArt]

Every time I'm saving post I'm getting browser alert.

I've deactivated all plugins - no issue, I've activated only Magic Fields - got this issue.

After big debug I've found that message is shown after code run:

Also in time of googling I've found the same problem at Advanced Custom Fields plugin:

1. https://support.advancedcustomfields.com/forums/topic/how-to-disable-js-alert-in-acf_form/
2. https://stackoverflow.com/questions/28295813/wordpress-disable-are-you-sure-you-want-to-navigate-away-from-this-page-whe

Hope, this will help.

WP 4.8.1, Magic Fields 2.1