Thanks to your bug report I think I have a fix for this rolled out now! I did a review of all calls to XMLHttpRequest in the project (they are all in components/session.js) and there was one place ...

What is AJAX (Asynchronous JavaScript and XML)? AJAX (Asynchronous JavaScript and XML) is a technique aimed at creating better and faster interactive web apps by combining several programming tools, ...

I've been debugging this a bit. It appears the problem is that when the XHR gets abort ()ed, it sets the properties.client to null. Then receiveResponse tries to use the client, but it doesn't exist.