Recommend `Optional::orElseThrow` instead of `get`

[gaul]

Java 10 recommends orElseThrow instead of get:

https://docs.oracle.com/javase/10/docs/api/java/util/Optional.html#get()

Some background:

http://mail.openjdk.java.net/pipermail/core-libs-dev/2016-April/040531.html