NPE thrown by deleted reviews in activity items

java.lang.NullPointerException
at com.cenqua.crucible.tags.ReviewUtil$PrincipalReviewKey.create(ReviewUtil.java:332)
at com.cenqua.crucible.tags.ReviewUtil$PrincipalReviewStringKey.create(ReviewUtil.java:352)
at com.cenqua.crucible.tags.ReviewUtil.principalCanDoReviewAction(ReviewUtil.java:162)
at com.atlassian.crucible.activity.review.LuceneReviewStateChangeActivityItemProvider.hasViewPermission(LuceneReviewStateChangeActivityItemProvider.java:138)
at com.atlassian.crucible.activity.review.LuceneReviewStateChangeActivityItemProvider.searchIndex(LuceneReviewStateChangeActivityItemProvider.java:126)
at com.atlassian.crucible.activity.review.LuceneReviewStateChangeActivityItemProvider.getMoreActivityItems(LuceneReviewStateChangeActivityItemProvider.java:110)
at com.atlassian.crucible.activity.review.LuceneReviewStateChangeActivityItemProvider.getMoreActivityItems(LuceneReviewStateChangeActivityItemProvider.java:37)
at com.atlassian.crucible.activity.review.CoalescingActivityItemProviderBase.findActivityItemsAndCoalesce(CoalescingActivityItemProviderBase.java:27)
at com.atlassian.crucible.activity.review.LuceneReviewStateChangeActivityItemProvider.findActivityItems(LuceneReviewStateChangeActivityItemProvider.java:54)
at com.atlassian.fisheye.activity.DefaultActivityItemManager.findActivityItems(DefaultActivityItemManager.java:49)
at com.atlassian.fisheye.activity.ActivityStreamHandler.handleActivityStream(ActivityStreamHandler.java:151)
at com.cenqua.fisheye.web.ChangelogServlet.doViewDir(ChangelogServlet.java:278)

We should purge deleted reviews from the index, but to avoid the NPE I added a defensive check (https://extranet.atlassian.com/crucible/changelog/FE/?cs=55336)