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'))) {