Fatal Error – payment not processed

After updating Woocommerce to 3.0.3, I’m unable to get payments. Orders without payment come in without problems, but paying clients get the following message after logging in to PayPal:

Fatal error: Call to a member function is_type() on null in /home/content/n3pnexwpnas02_data01/12/3539712/html/wp-content/plugins/woocommerce/includes/class-wc-cart.php on line 543

I thought it was a problem with the theme. Changed theme but that didn’t solve the problem.

Orders appear in the backend as “pending payment”. The client sees the fatal error message on the screen.