Corridor Surface Creation using it's featurelines

Hi, I was trying to create Corriodor surface using the feature lines by using Surface.ByLoft node in which my “crosssection” input will be my corridor feature lines.
Initially it was working fine [there was even surface created in the 3d preview]
but after few steps it showed up error as shown in the picture with null output, initally there were two outputs (one was ETW left and the other was ETW right). What would be possibly causing this error?
I was trying to create the second script mentioned in this video