Payment Confirmation Error

We are seeing an issue very similar to that described here:

https://wpinvoicing.com/support/topic/issue-with-stripe/

We are using PayPal Pro as the payment gateway. When a customer adds their listing to our new therapist directory site, everything works except the final confirmation screen. The payment gets made at PayPal, the listing gets saved, email notifications work. But at the end of the process we are getting a screen like this:

This page isn’t working

therapyforblackmen.press is currently unable to handle this request.
HTTP ERROR 500

The URL for this screen is:

https://therapyforblackmen.press/wpi-checkout/

I have found these Apache log entries:

99.252.35.84 – – [03/Jun/2018:14:44:23 +0000] “POST /wpi-checkout/ HTTP/1.0” 500 296 “https://therapyforblackmen.press/wpi-checkout/” “Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.181 Safari/537.36″

[Sun Jun 03 14:13:16.403612 2018] [proxy_fcgi:error] [pid 7319] [client 127.0.0.1:60190] AH01071: Got error ‘PHP message: PHP Fatal error: Uncaught Error: Call to undefined method WPInv_Invoice::get_new_expiration() in /home/175185.cloudwaysapps.com/knazvbhxtv/public_html/wp-content/plugins/wpinv-paypal-pro-payment/includes/class-invoicing-paypal-pro.php:520\nStack trace:\n#0 /home/175185.cloudwaysapps.com/knazvbhxtv/public_html/wp-content/plugins/wpinv-paypal-pro-payment/includes/class-invoicing-paypal-pro.php(465): PaypalPro->wpinv_paypalpro_record_subscription_signup(Array, Object(WPInv_Invoice))\n#1 /home/175185.cloudwaysapps.com/knazvbhxtv/public_html/wp-includes/class-wp-hook.php(286): PaypalPro->process_paypal_call(Array)\n#2 /home/175185.cloudwaysapps.com/knazvbhxtv/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array)\n#3 /home/175185.cloudwaysapps.com/knazvbhxtv/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array)\n#4 /home/175185.cloudwaysapps.com/knazvbhxtv/public_html/wp-content/plugins/invoicing/includes/wpinv-gateway-functions.php(265): do_action(‘wpinv_gateway…\n’, referer: https://therapyforblackmen.press/wpi-checkout/