java.lang.NullPointerException: sessionId should not be null; has this session been started yet?
   at org.openqa.selenium.server.FrameGroupCommandQueueSet.getQueueSet(FrameGroupCommandQueueSet.java:220)
   at org.openqa.selenium.server.commands.SeleniumCoreCommand.execute(SeleniumCoreCommand.java:37)
   at org.openqa.selenium.server.SeleniumDriverResourceHandler.doCommand(SeleniumDriverResourceHandler.java:611)
   at org.openqa.selenium.server.SeleniumDriverResourceHandler.handleCommandRequest(SeleniumDriverResourceHandler.java:405)
   at org.openqa.selenium.server.SeleniumDriverResourceHandler.handle(SeleniumDriverResourceHandler.java:149)
   at org.openqa.jetty.http.HttpContext.handle(HttpContext.java:1530)
   at org.openqa.jetty.http.HttpContext.handle(HttpContext.java:1482)
   at org.openqa.jetty.http.HttpServer.service(HttpServer.java:909)
   at org.openqa.jetty.http.HttpConnection.service(HttpConnection.java:820)
   at org.openqa.jetty.http.HttpConnection.handleNext(HttpConnection.java:986)
   at org.openqa.jetty.http.HttpConnection.handle(HttpConnection.java:837)
   at org.openqa.jetty.http.SocketListener.handleConnection(SocketListener.java:243)
   at org.openqa.jetty.util.ThreadedServer.handle(ThreadedServer.java:357)
   at org.openqa.jetty.util.ThreadPool$PoolThread.run(ThreadPool.java:534)