Fix for crash on ask dialog

This failed from e6eac8680fdd100dd15103b7ffcf114ff6bf1784 which was rubocop introduced to turn an ivar access into an attr_reader. Unfortunately this was within our nested `Shoes.app` call, so the scoping is weird and `attr_reader` is actually undefined there.

Stepping into the singleton class lets us use `attr_reader` properly, without rubocop complaining.

Possible alternative--we could implement attr_reader/writer/accessor in `Shoes::InternalApp` to delegate properly to the singleton class of the app instance, which would let people just use them directly. I think I recall someone mentioning before that they couldn't use `attr_reader`, and this would fix that and probably meet people's expectations better. Thoughts @PragTob @wasnotrice @KCErb?