Unable to load sitemap data in concrete 5.4.1.1

Best solution without upgrading to the next version from andrew in the c5 forums.  Change method_exists to is_callable in the iterm_list.php. You can view the code here, on github.

Scroll down to the code block for web/concrete/libraries/item_list.php to see where line 106 has been replaced.

 if (method_exists($this->attributeClass, 'getList')) {

changes to:

   if (is_callable(array($this->attributeClass, 'getList'))) {