Showing the following error:Warning: Use of undefined constant WPDEVART_PLUGIN_U

When I enter the settings for booking calendar I get the following error:

Warning: Use of undefined constant WPDEVART_PLUGIN_URL – assumed ‘WPDEVART_PLUGIN_URL’ (this will throw an Error in a future version of PHP) in C:\xampp\htdocs\wordpress\wp-content\plugins\booking-calendar\includes\wpdevart_lib.php on line 50