Solidus Version:
all, including master

How to reproduce
Trying to empty a shipped order via API raises a 500 error, which is something we should try to avoid.

This can be easily demonstrated in the admin area:

• on the Cart tab, add an item to a new order, and keep this browser window as it is, we'll get back to it later;
• add the address, payment, and finalize the order in a different browser window/tab;
• ship the order;
• go back to the Cart browser window and click the Empty Cart button. Web developer tools will show a 500 error, and no message is presented to the user.

Context

This is the method definition from the API orders controller:

      def empty
        authorize! :update, @order, order_token
        @order.empty!
        render plain: nil, status: 204
      end

@order.empty! tries to reset the cart by destroying the associated records, but shipped shipments cannot be destroyed. The error bubbles up to the controller, where it's not handled.