Fix a problem with a wordpress plugin

We have a problem with a file, [login to view URL] on line 210 : the length of the variable cannot be read because the element is null. The checkout process has been modified to behave a certain way with iFrames. To be more specific, the files [login to view URL] and [login to view URL] were changed to allow the Paypal gateway payment button to open a new tab to bypass the X-Frame-Options limitations enforced by Paypal. The symptom of the problem is that sometimes, randomly, we won't be able to go through the checkout process when viewing the pages through an iframe : [login to view URL] I have found that the best way to reproduce this error is to be logged out of Facebook, in Chrome's incognito mode (your cart needs to be empty), go to this URL : [login to view URL] Then pick a product and click on the "Add To Order" button. Hopefully you'll have the same issue shown in the screenshot. If not you'll have to try a few times and eventually you'll get the error. When you get the error, if you try to add more products to the order, that's where you will see in Chrome's console the [login to view URL] error on line 210. We need to find a fix to prevent this error from happening. I have added all required files, normally. Let me know if you need anything else.