Undefined $extra_url

Hi I’m testing the plugin to see if is going to work for my project and buy the pro version, so far it’s been perfect, I just have one problem.
Im testing the paypal functionality with a sandbox but after the payment is done, and try to redirect I got this error:
Undefined index: extra_url …. on booking-system/addons/paypal/dopbsp-paypal.php on line 198

I’ve been checking the code, but I’m not able to find where do you get that variable, what is extra_url for ? Will i have problems if I set some ifs to check if the parameter exists ?

https://ww.wp.xz.cn/plugins/booking-system/