The files

admin/includes/menu.css
admin/includes/menu.js
admin/includes/stylesheet.css

will no longer be part of Zen Cart after 1.5.8b is released. Please see

https://docs.zen-cart.com/dev/plugin..._head_content/

for details.

Please remove them from your contributions (as described on the FAQ page above) for Zen Cart 1.5.7 and above, or modify them as shown in post #4 below if you wish your plugin to work in old and new versions of Zen Cart.