Won’t save block; fatal error

Hi all. One of our site administrators noticed that they could not save any changes to blocks. After turning debugging on for the site, I noticed the following:

[17-Oct-2018 15:32:41 UTC] PHP Fatal error:  Uncaught Error: Undefined class constant 'DST_FILTER_OPTIONS' in /home/kkclient/healthy-holistic-living.com/wp-content/plugins/ad-inserter/ad-inserter.php:6952
Stack trace:
#0 /home/kkclient/healthy-holistic-living.com/wp-settings.php(305): include_once()
#1 /home/kkclient/healthy-holistic-living.com/wp-config.php(119): require_once('/home/kkclient/...')
#2 /home/kkclient/healthy-holistic-living.com/wp-load.php(37): require_once('/home/kkclient/...')
#3 /home/kkclient/healthy-holistic-living.com/wp-blog-header.php(13): require_once('/home/kkclient/...')
#4 /home/kkclient/healthy-holistic-living.com/index.php(17): require('/home/kkclient/...')
#5 {main}
  thrown in /home/kkclient/healthy-holistic-living.com/wp-content/plugins/ad-inserter/ad-inserter.php on line 6952

The code in question is:

// add_filter (DST_Client::DST_FILTER_OPTIONS . AD_INSERTER_SLUG, 'ai_dst_options');

I had to comment it out in order for the plugin to stop giving the WSOD. After commenting it out, the blocks saves still fail, but the plugin does insert the ads that were previously defined.

The page I need help with: [log in to see the link]