当使用抛出的NSCoding错误将对象解码为Int时是:致命错误:在打开可选值时意外发现nil

I have a a set of objects in Swift which I am using NSCoding to save to disk so that they can be read into memory the next time the app is run. The item being saved is an array of Volume objects. Each instance of Volume has a volumeNumber Int which is set. Both the Volume and QA class inherit from NSObject and NSCoding.I have a a set of objects in Swift which I am u