Nick:
Object Reference errors are the most elusive / irritating - because you cannot discern much from the error other than there is a missing "something."
If your xml works via Portal, have a look at the following post. It details some changes we are making with Data Exchange. The Portal interface is currently consuming the API through "new" URL (referenced in the post). Try changing your code to point to the new URL and see if that fixes it for you.
If not, email your code to api[at]fellowshiptech.com and the team will have a look at it and help you get it up and running.
I hope this helps