internalConfiguration.ServerURL is not being updated as initialize() method is never called (at least in code you've provided.) ServerURL field is empty by default so that is why it's null in httpservice. Take into account that ServerURL should be bindable. Also ServerURL should has setter, otherwise binding will not be triggered.
And don't forget that config has to be bindable either:
[Bindable]
private var internalConfiguration:AppConfiguration = AppConfiguration.getInstance();