admin-ajax.php Fatal Error

I'm getting a PHP Fatal error that looks like this:

Error Message:

PHP message: PHP Fatal error:  Uncaught 
Error: Call to a member function get_children() on null in 
/www/wp-content/plugins/wpdiscuz/class.WpdiscuzCore.php:1633
Stack trace:
#0 /www/wp-includes/class-wp-hook.php(287): 
WpdiscuzCore->mostReactedComment('')
#1 /www/wp-includes/class-wp-hook.php(311): 
WP_Hook->apply_filters('', Array)
#2 /www/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#3 /www/wp-admin/admin-ajax.php(190): do_action('wp_ajax_nopriv_...')
#4 {main}
  thrown 

Any ideas why?