Github user khalidhuseynov commented on the issue:
https://github.com/apache/zeppelin/pull/1338
somehow i keep getting this error when doing checkpoint:
```
ERROR [2016-08-22 14:33:04,618] ({qtp787867107-76}
NotebookServer.java[onMessage]:257) - Can't handle message
java.lang.NullPointerException
at org.eclipse.jetty.client.HttpClient.send(HttpClient.java:521)
at org.eclipse.jetty.client.HttpRequest.send(HttpRequest.java:694)
at org.eclipse.jetty.client.HttpRequest.send(HttpRequest.java:678)
at
org.apache.zeppelin.notebook.repo.zeppelinhub.rest.ZeppelinhubRestApiHandler.sendToZeppelinHub(ZeppelinhubRestApiHandler.java:157)
at
org.apache.zeppelin.notebook.repo.zeppelinhub.rest.ZeppelinhubRestApiHandler.asyncPutWithResponseBody(ZeppelinhubRestApiHandler.java:125)
at
org.apache.zeppelin.notebook.repo.zeppelinhub.ZeppelinHubRepo.checkpoint(ZeppelinHubRepo.java:201)
at
org.apache.zeppelin.notebook.repo.NotebookRepoSync.checkpoint(NotebookRepoSync.java:334)
at
org.apache.zeppelin.notebook.Notebook.checkpointNote(Notebook.java:358)
at
org.apache.zeppelin.socket.NotebookServer.checkpointNotebook(NotebookServer.java:1182)
at
org.apache.zeppelin.socket.NotebookServer.onMessage(NotebookServer.java:233)
at
org.apache.zeppelin.socket.NotebookSocket.onWebSocketText(NotebookSocket.java:56)
at
org.eclipse.jetty.websocket.common.events.JettyListenerEventDriver.onTextMessage(JettyListenerEventDriver.java:128)
at
org.eclipse.jetty.websocket.common.message.SimpleTextMessage.messageComplete(SimpleTextMessage.java:69)
at
org.eclipse.jetty.websocket.common.events.AbstractEventDriver.appendMessage(AbstractEventDriver.java:65)
at
org.eclipse.jetty.websocket.common.events.JettyListenerEventDriver.onTextFrame(JettyListenerEventDriver.java:122)
at
org.eclipse.jetty.websocket.common.events.AbstractEventDriver.incomingFrame(AbstractEventDriver.java:161)
at
org.eclipse.jetty.websocket.common.WebSocketSession.incomingFrame(WebSocketSession.java:309)
at
org.eclipse.jetty.websocket.common.extensions.ExtensionStack.incomingFrame(ExtensionStack.java:214)
at
org.eclipse.jetty.websocket.common.Parser.notifyFrame(Parser.java:220)
at org.eclipse.jetty.websocket.common.Parser.parse(Parser.java:258)
at
org.eclipse.jetty.websocket.common.io.AbstractWebSocketConnection.readParse(AbstractWebSocketConnection.java:632)
at
org.eclipse.jetty.websocket.common.io.AbstractWebSocketConnection.onFillable(AbstractWebSocketConnection.java:480)
at
org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)
at
org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
at
org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
at java.lang.Thread.run(Thread.java:745)
```
not sure what's the problem though, let me know if doing something wrong
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