Compatibility issue with 4.9.9

Hi all, this morning I discovered a compatibility issue of this plugin with wordpress 4.9.9. If I try to save a new page for example the following error is generated:

PHP Fatal error:  Uncaught Error: Call to undefined function wp_is_json_request() 
in /var/www/mysite.it.alfa4/wp-content/plugins/advanced-custom-fields/includes/l10n.php:34\n
Stack trace:\n#0 /var/www/mysite.it.alfa4/wp-content/plugins/advanced-custom-fields/includes/l10n.php(67): determine_locale()\n
#1 /var/www/mysite.it.alfa4/wp-content/plugins/advanced-custom-fields/includes/l10n.php(118): acf_get_locale()\n
#2 /var/www/mysite.it.alfa4/wp-content/plugins/advanced-custom-fields/acf.php(253): acf_load_textdomain()\n
#3 /var/www/mysite.it.alfa4/wp-includes/class-wp-hook.php(286): ACF->init('')\n
#4 /var/www/mysite.it.alfa4/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array)\n
#5 /var/www/mysite.it.alfa4/wp-includes/plugin.php(453): WP_Hook->do_action(Array)\n
#6 /var/www/mysite.it.alfa4/wp-settings.php(450): do_action('init')\n
#7 /var/www/mysite.it.alfa4/wp-config.php(94): require_once('/var/www/ortige...')\n
#8 /var/www/mysite.it.alfa4/wp-load.php(37): require_onc in /var/www/mysite.it.alfa4/wp-content/plugins/advanced-custom-fields/includes/l10n.php on line 34, 
referer: https://www.mysite.it/wp-admin/post.php?post=4639&action=edit

Temporary workaround is to add the function to my theme:

function wp_is_json_request() {
    if ( isset( $_SERVER['HTTP_ACCEPT'] ) && false !== strpos( $_SERVER['HTTP_ACCEPT'], 'application/json' ) ) {
        return true;
    }
    if ( isset( $_SERVER['CONTENT_TYPE'] ) && 'application/json' === $_SERVER['CONTENT_TYPE'] ) {
        return true;
    }
    return false;
}

Hope this helps, Thanks.