Thanks to Imron.
id was set in the arguments array for the "Sidebar 1" sidebar. Defaulting to "sidebar-1". Manually set the id to "sidebar-1" to silence this notice and keep existing sidebar content. Please see Debugging in WordPress for more information. (This message was added in version 4.2.0.) in /var/www/vhosts/signese.com/httpdocs/wp-includes/functions.php on line 5173
Thanks to Imron.