While trying to get the test in Varun's patch in
SOLR-5750 (SolrCloud collection backup & restore) to succeed, I kept hitting an NPE in Overseer in which clusterState was null. I added a bunch of asserts and found where it was happening which I worked around temporarily. See https://github.com/apache/lucene-solr/commit/fd9c4d59e8dbe0e9fbd99a40ed2ff746c1ae7a0c#diff-9ed614eee66b9e685d73446b775dc043R247 which is ZkStateWriter.writePendingUpdates() returning null, overwriting the current non-null clusterState. This happens nearly every time for me when I run that test.