Checkout: Errors when removing item from sidebar cart

[sirbrillig]

Uncaught TypeError: Cannot read property 'product_name' of undefined

To reproduce:

1. Add an upgrade to your cart for a new site or an existing one. e.g.: visit the plans page for an existing site (https://wordpress.com/plans/example.wordpress.com) and click "Upgrade Now".
2. Click the "x" next to the upgrade in the Checkout page sidebar.
3. The page hangs on a loading screen and errors appear in the console.
4. The error persists upon reloading the Checkout page.

In screenshots:

[gziolo]

I was able to reproduce this one only when I had free credits on my account.