Error "Use of undefined constant ABD_Widget" appears

Hello,

thank you for your great plugin. If I use that plugin, I get the following error:
Use of undefined constant ABD_Widget - assumed 'ABD_Widget' in \wp-content\plugins\ad-blocking-detector\includes\widget.php on line 9

If I change the code of this line from
if( !class_exists( ABD_Widget ) ) {
to
if( !class_exists( 'ABD_Widget' ) ) {
it works.

Can you add that fix to your next version? Thank you!

Regards, Kristian

https://www.ads-software.com/plugins/ad-blocking-detector/