In certain situations, Chrome spits the error mentioned in the title.
I tracked it down to an insufficient assignment (could probably be fixed earlier in the flow) and found a fix which I posted on stackoverflow already: http://stackoverflow.com/questions/9449879#10037534
Since this seems to be an actual bug in MooTools Core, I thought I'd report here too.
So that is this line: https://github.com/mootools/mootools-core/blob/master/Source/Fx/Fx.CSS.js#L106
Could we narrow it down when exactly those are null?
Closed as per #2386