Combined paid and free membership price sets give an error when free membership is selected

Occurs when you have a price set that contains both paid and free memberships on a contribution page, where there is a payment processor set as default. When the free membership is selected, the payment method section (payment processors and the pay by check) disappears, and when they confirm the transaction, CiviCRM is giving an error that the API URL is required.

Workaround - the customer has to scroll down and select the Pay by Check. Then scroll back up and select the free membership. That removes the need for the API.

A solution would either 1) (best) remove the requirement for a payment processor API if the amount = $0. Or 2) (not so good) set the payment process to Pay by Check when the free membership is selected (when the Payment Processor selection disappears). Even if free memberships select Pay by Check, then the admin would have to complete the contribution from Pending to Complete.