When you add an item then delete it from the cart, the products list is broken.

A critical error has occurred.

Deleted row information cannot be accessed through the row.

At this point even logging out does not correct the problem. An application restart is required to fix it.

UPDATE: This appears to be a one shot upgrade issue and resolves itself which may indicate stale cookies. It is unknown if this affects every customer with cookies from versions prior to 4.21.