Here is the error I am seeing: Error: item is null Source File: https://builder.addons.mozilla.org/media/lib/mootools/mootools-core-1.3.2.js Line: 936
Just save a bunch back and forth between many tabs and mix in some test button hits to trigger this.
Similar with this one, is there anymore specific of a situation that you could provide? So far, I can't reproduce this.
However, I did run into another error while testing this (that if you closed the first tab on the left, you could then close all the tabs). It turned out that closing the first tab only destroyed the element, but left the editor instance around. That could have been the reason this was happening as well.
If this can no longer be reproduced, then I'm going to say it was related to the other bug and is now fixed.