fix(checkout): CHECKOUT-7048 Allow user to clear cart changed error

[huntario]

What?

... Check to see if body is defined within the handleCloseModal method

Why?

... When it it undefined the user cannot clear the error to complete checkout - https://bigcommercecloud.atlassian.net/browse/CHECKOUT-7048

Testing / Proof

... Before: 0:25 error cannot be cleared - TypeError: Cannot read properties of undefined (reading 'type')

https://user-images.githubusercontent.com/5630999/198082107-7c2234dd-e92a-4806-befa-a87882be4fc4.mp4

After: 0:24 error can be cleared to complete checkout

https://user-images.githubusercontent.com/5630999/198083790-1dfa816b-3af5-4bda-b717-0ccf54b5431e.mp4

@bigcommerce/checkout