It looks like the root issue is that we are not getting a response from the REST API. This is because until the build get's an executor it won't have a REST API page. whenw e try and call the page we get a null or response. There is an update coming for the JSON issue you're seeing, thanks to @scotthains, which will likely fix your issue.
I think this should work because getBuildStatus method takes a null response (from sendHTTPCall) to mean the build has not yet started - the problem with this though is that this means if someone cancels the job before it gets an executor the Remote trigger plugsdsdin will wait indefinitely.
The best way I can see to solve this is to try and get Jenkins Core to give jobs an API page before they get an executor. I'm not sure how easy that would be as, if I remember correctly, the job is a different class of object after it gets and executor and before.
Did you check with the link Maurice posted?