ReferenceError wp is not defined

When I enable WPUSSC Firebug catches an error when I load the Dashboard page:
ReferenceError: wp is not defined.

The error is caught in line 499 of wp-admin/index.php.
wp.media.editor.remove(‘content’);

If I deactivate the WPUSSC it goes away.

Now, line 499 is a bit of javascript. I cannot find any reference in WPUSSC that might be a match.

What might be going on?

https://www.ads-software.com/extend/plugins/wp-ultra-simple-paypal-shopping-cart/