[SPARK-2643] Stages web ui has ERROR when pool name is None

14/07/23 16:01:44 WARN servlet.ServletHandler: /stages/
java.util.NoSuchElementException: None.get
at scala.None$.get(Option.scala:313)
at scala.None$.get(Option.scala:311)
at org.apache.spark.ui.jobs.StageTableBase.stageRow(StageTable.scala:132)
at org.apache.spark.ui.jobs.StageTableBase.org$apache$spark$ui$jobs$StageTableBase$$renderStageRow(StageTable.scala:150)
at org.apache.spark.ui.jobs.StageTableBase$$anonfun$toNodeSeq$1.apply(StageTable.scala:52)
at org.apache.spark.ui.jobs.StageTableBase$$anonfun$toNodeSeq$1.apply(StageTable.scala:52)
at org.apache.spark.ui.jobs.StageTableBase$$anonfun$stageTable$1.apply(StageTable.scala:61)
at org.apache.spark.ui.jobs.StageTableBase$$anonfun$stageTable$1.apply(StageTable.scala:61)
at scala.collection.immutable.Stream$$anonfun$map$1.apply(Stream.scala:376)
at scala.collection.immutable.Stream$$anonfun$map$1.apply(Stream.scala:376)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1085)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1077)
at scala.collection.immutable.StreamIterator$$anonfun$next$1.apply(Stream.scala:980)
at scala.collection.immutable.StreamIterator$$anonfun$next$1.apply(Stream.scala:980)
at scala.collection.immutable.StreamIterator$LazyCell.v$lzycompute(Stream.scala:969)
at scala.collection.immutable.StreamIterator$LazyCell.v(Stream.scala:969)
at scala.collection.immutable.StreamIterator.hasNext(Stream.scala:974)
at scala.collection.Iterator$class.foreach(Iterator.scala:727)
at scala.collection.AbstractIterator.foreach(Iterator.scala:1157)
at scala.xml.NodeBuffer.$amp$plus(NodeBuffer.scala:38)
at scala.xml.NodeBuffer.$amp$plus(NodeBuffer.scala:40)
at org.apache.spark.ui.jobs.StageTableBase.stageTable(StageTable.scala:60)
at org.apache.spark.ui.jobs.StageTableBase.toNodeSeq(StageTable.scala:52)
at org.apache.spark.ui.jobs.JobProgressPage.render(JobProgressPage.scala:91)
at org.apache.spark.ui.WebUI$$anonfun$attachPage$1.apply(WebUI.scala:65)
at org.apache.spark.ui.WebUI$$anonfun$attachPage$1.apply(WebUI.scala:65)
at org.apache.spark.ui.JettyUtils$$anon$1.doGet(JettyUtils.scala:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:684)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:501)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:428)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:255)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
at org.eclipse.jetty.server.Server.handle(Server.java:370)
at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:494)
at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:971)
at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:1033)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:644)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:667)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
at java.lang.Thread.run(Thread.java:744)
14/07/23 16:01:44 WARN server.AbstractHttpConnection: /stages/
java.lang.NoSuchMethodError: javax.servlet.http.HttpServletRequest.isAsyncStarted()Z
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:583)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:428)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:255)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
at org.eclipse.jetty.server.Server.handle(Server.java:370)
at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:494)
at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:971)
at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:1033)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:644)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:667)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
at java.lang.Thread.run(Thread.java:744)
This is because:
(1) One stage for example the stage 10 has been completed and then it's resubmitted. 
(2) Then JobProgressListener.trimIfNecessary is triggered by another SparkListenerStageCompleted event. The StageUIData of the stage 10 will be removed from memory although the stage 10 is active at present.
(3) JobProgressPage will not get the StageUIData of the stage 10 any more.
Does it matter than the first `toRemove` elements now may not have been removed? I think this has to be consistent with the previous line.