Apologies in advance if this is an obvious answer but I've searched and can't locate one.

I'm trying to follow the 'Sofa and Cushions' book and having an issue with Soap Bubble. It appears installed OK as I have a toolbar and can create a skin. But when I try to do anything with the skin I get:

Error: #<NoMethodError: undefined method <=>' for nil:NilClass> (eval):309:in onUserText'
(eval):309:in sort!' (eval):309:in onUserText'

I feel I'm missing something fundamental?

Thanks!