title: OpsCenter 6.1.2 keeps crashing after update from 6.0.X "No agents available to query for getDsePerformanceStatusFromOneAgent"
body: <p>After updating OpsCenter 6.0.X to 6.1.2 using yum, OpsCenter frequently crashes with the following error in opsenterd.log:</p>
  
  <pre><code>2017-08-25 10:03:34,066 [opscenterd] ERROR: Unhandled error in Deferred: No agents available to query for getDsePerformanceStatusFromOneAgent.
  File "/usr/share/opscenter/lib/py/twisted/internet/defer.py", line 1124, in _inlineCallbacks
  result = g.send(result)
  File "/usr/share/opscenter/jython/Lib/site-packages/opscenterd/Cluster.py", line 1301, in callOnAgent
  </code></pre>
  
  <p>(MainThread)</p>
  
  <p>Any idea how to fix this?</p>