Fix Vanilla JS bug on cart page

[nielslange]

Steps to reproduce

• Add a product to the cart
• Go to the cart page
• Update the quantity
• See error

Expected behavior

The quantity can be changed.

Current behavior

The quantity cannot be changed.

Reported in

• [ ] https://wordpress.org/support/topic/version-1-20-is-broken/
• [ ] https://wordpress.org/support/topic/js-error-in-v1-2/
• [ ] https://wordpress.org/support/topic/update-cart-after-increasing-or-decreasing-quantity/
• [ ] https://wordpress.org/support/topic/site-broken-after-trying-to-remove-plugin/
• [ ] https://wordpress.org/support/topic/the-free-version-is-now-useless-2/