Throw NullPointerException on Planning Board when Achive Master Release in JIRA with keeping Sub Release Unachived

Symptom
If user achives master release in JIRA and keeps sub release(s) unachived, Planning Board will throw NullPointerException when selecting that project.

Cause
com.pyxis.greenhopper.jira.util.comparator.UnreleasedVersionWeightResolver can not find master release for sub release(s) in the logic of getWeight() method, then throw NullPointerException.

Workaround
Unachiving the relevant master version will resolve the problem.

Resolution
In my test, this problem can't be reproduced in GreenHopper 4.0+