Which line of the arc script throws this error?

Retrieving dimension values during the startup is broken right now... because they are not really set. They should just be nil.
However parent should not be nil... unless... there is a now kind of obsolete subclass called `AbsoluteDimensions` which is used for arc and co - basically all art elements. It shouldn't break though. There should be guards and stuff...

right now your branch also seems to be behind master significantly as you still got the old version of dimensions.