Fix NPE in Master WebUI after recoverying Master

Author: zsxwing

core/src/main/scala/org/apache/spark/deploy/master/ApplicationInfo.scala

@@ -66,6 +66,7 @@ private[spark] class ApplicationInfo(
     nextExecutorId = 0
     removedExecutors = new ArrayBuffer[ExecutorDesc]
     executorLimit = Integer.MAX_VALUE
+    appUIUrlAtHistoryServer = None
   }
 
   private def newExecutorId(useID: Option[Int] = None): Int = {

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -690,7 +690,7 @@ private[deploy] object Worker extends Logging {
     val conf = new SparkConf
     val args = new WorkerArguments(argStrings, conf)
     val rpcEnv = startRpcEnvAndEndpoint(args.host, args.port, args.webUiPort, args.cores,
-      args.memory, args.masters, args.workDir)
+      args.memory, args.masters, args.workDir, conf = conf)
     rpcEnv.awaitTermination()
   }