Notice: The called constructor method for WP_Widget

My client’s website http://mansourmusic.com is running MU Type theme (which is no longer available or supported) but isn’t functioning properly since the 4.5 update. I’ve contacted the theme developer who said they’re not updating or supporting the theme anymore, but suggested I debug the issue. The following message is what I got:

Notice: The called constructor method for WP_Widget in Ads_Widget is deprecated since version 4.3.0! Use
__construct()
instead. in /home/content/p3nexnas05_data02/07/2176007/html/wp-includes/functions.php on line 3722

I’ve deactivated all plugins and the issue still persists. I know it’s the theme, but does anyone have any experience with this and how to fix it? Any help or suggestions will be greatly appreciated!