SPARK-1965 [WEBUI] Spark UI throws NPE on trying to load the app page for non-existent app

Don't throw NPE if appId is unknown. kayousterhout is this a decent enough band-aid for avoiding a full-blown NPE? it should just render empty content instead

Author: Sean Owen <sowen@cloudera.com>

Closes #4777 from srowen/SPARK-1965 and squashes the following commits:

7e16590 [Sean Owen] Update app not found message
cb878d6 [Sean Owen] Return basic "not found" page for unknown appId
d8270da [Sean Owen] Don't throw NPE if appId is unknown

core/src/main/scala/org/apache/spark/deploy/master/ui/ApplicationPage.scala

@@ -24,6 +24,7 @@ import scala.xml.Node
 
 import akka.pattern.ask
 import org.json4s.JValue
+import org.json4s.JsonAST.JNothing
 
 import org.apache.spark.deploy.{ExecutorState, JsonProtocol}
 import org.apache.spark.deploy.DeployMessages.{MasterStateResponse, RequestMasterState}
@@ -44,7 +45,11 @@ private[spark] class ApplicationPage(parent: MasterWebUI) extends WebUIPage("app
     val app = state.activeApps.find(_.id == appId).getOrElse({
       state.completedApps.find(_.id == appId).getOrElse(null)
     })
-    JsonProtocol.writeApplicationInfo(app)
+    if (app == null) {
+      JNothing
+    } else {
+      JsonProtocol.writeApplicationInfo(app)
+    }
   }
 
   /** Executor details for a particular application */
@@ -55,6 +60,10 @@ private[spark] class ApplicationPage(parent: MasterWebUI) extends WebUIPage("app
     val app = state.activeApps.find(_.id == appId).getOrElse({
       state.completedApps.find(_.id == appId).getOrElse(null)
     })
+    if (app == null) {
+      val msg = <div class="row-fluid">No running application with ID {appId}</div>
+      return UIUtils.basicSparkPage(msg, "Not Found")
+    }
 
     val executorHeaders = Seq("ExecutorID", "Worker", "Cores", "Memory", "State", "Logs")
     val allExecutors = (app.executors.values ++ app.removedExecutors).toSet.toSeq