WordPress with tinymce trying open not existing image in loop

Hello,

I think it happens because of Heartbeat API:

Heartbeat API allows your browser to communicate with the server when you’re logged into the WordPress admin panel. This functionality allows WordPress to handle things like showing other authors that a post is being edited by another user, plugins can hook up to those “ticks” and show you notifications in real time, etc.

you can disable heartbeat completely by adding the following code into your theme’s functions.php file:

add_action( 'init', 'stop_heartbeat', 1 );
function stop_heartbeat() {
wp_deregister_script('heartbeat');
}

or you can control heartbeat’s interval with Heartbeat Control plugin.
Let me know if it helps.

Thanks