Replace item at index from 2 list

Hi i am new here. i am trying to get areas from windows and doors edges and laterset value in walls. first I separate windows and doors get their edges and do a multiplication by their thickness. i wanted to do it all in one operation but when firts item was empty i get an error. so i separate the lists and later join them using replace item at index but dynamo doesnt do it how i expect i have a image i hope u can help me

I’d need to see more of you inputs/outputs to work out what’s gone wrong - but an easy solution could be to use MergeByBoolMask from the Springs package.

thanks Hamish I think I got it but in another way.