Take a look at this blog post on how to set up Grid2 https://goo.gl/rxzbE6. A user related (in the email group on the source) that using older selenium libs solves this exception.
You have a different solution? A short tip here would help you and many other users who saw this issue last week.
- java.lang.NullPointerException: inputStream cannot be null at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:204)