The Jenkins master has been running for about 2 months now and the Gerrit trigger plug-in has been working well.
We see these NPE problems with intervals (I have attached the log) after we see the Gerrit Trigger no longer being able to connect to the Gerrit server, so we try to restart the Gerrit Trigger:
Dec 17, 2013 10:26:37 AM com.sonyericsson.hudson.plugins.gerrit.trigger.GerritManagement$DescriptorImpl doRestartConnection
SEVERE: Could not restart connection.
java.lang.NullPointerException
at com.sonyericsson.hudson.plugins.gerrit.gerritevents.watchdog.StreamWatchdog.shutdown(StreamWatchdog.java:131)
at com.sonyericsson.hudson.plugins.gerrit.gerritevents.GerritHandler.shutdown(GerritHandler.java:850)
at com.sonyericsson.hudson.plugins.gerrit.trigger.PluginImpl.stopConnection(PluginImpl.java:283)
at com.sonyericsson.hudson.plugins.gerrit.trigger.PluginImpl.restartConnection(PluginImpl.java:299)
at com.sonyericsson.hudson.plugins.gerrit.trigger.GerritManagement$DescriptorImpl.doRestartConnection(GerritManagement.java:154)

In the Gerrit Trigger plug-in page it says this:
"The connection to Gerrit is down! Check your settings and the Gerrit server.
Since the connection is down, Jenkins cannot determine the Gerrit version. So all Gerrit features have been enabled."

I can, however, easily press the "Test connection" button and have that work.