如何使用Android KeyStore安全存储任意字符串？

I would like to be able securely store some sensitive strings in the Android KeyStore. I get the strings from the server but I have a use case which requires me to persist them. KeyStore will only allow access from the same UID as that assigned to my app, and it will encrypt the data with the device master password, so it's my understanding that I don't have to do any additional encryption to protect my data. My trouble is, I'm missing something about how to write the data. The code I have below works perfectly, as long as the call to KeyStore.store(null) is omitted. That code fails, and as long as I can't store the data after putting it to the KeyStore, then I can't persist it.I would like to be able securely store some sen