I'll be uploading an updated version of the plugin in the next few days; until then, here's a fix:
In the "accordion_menu_walker.php" file in the plugin folder, find the following code:
class Accordion_Menu_Walker extends Walker_Nav_Menu{
function display_element ($element, &$children_elements, $max_depth, $depth = 0, $args, &$output){
And change it to:
class Accordion_Menu_Walker extends Walker_Nav_Menu{
var $options;
function display_element ($element, &$children_elements, $max_depth, $depth = 0, $args, &$output){
That should fix all the errors. Let me know ASAP if there are any other issues with the plugin, as I will only be working on updating it today and tomorrow.
If you don't want to add that single line of code, you can wait until Wednesday and just grab the latest download of the plugin, as I plan to have the download updated by then.