[SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes

JobProgressPage could not show Fair Scheduler Pools section sometimes.
SparkContext starts webui and then postEnvironmentUpdate. Sometimes JobProgressPage is accessed between webui starting and postEnvironmentUpdate, then the lazy val isFairScheduler will be false. The Fair Scheduler Pools section will not display any more.

Author: yantangzhai <tyz0303@163.com>
Author: YanTangZhai <hakeemzhai@tencent.com>

Closes #1966 from YanTangZhai/SPARK-3067 and squashes the following commits:

d4323f8 [yantangzhai] update [SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes
8a00106 [YanTangZhai] Merge pull request #6 from apache/master
b6391cc [yantangzhai] revert [SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes
d2226cd [yantangzhai] [SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes
cbcba66 [YanTangZhai] Merge pull request #3 from apache/master
aac7f7b [yantangzhai] [SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes
cdef539 [YanTangZhai] Merge pull request #1 from apache/master

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -238,7 +238,6 @@ class SparkContext(config: SparkConf) extends Logging {
       // For tests, do not enable the UI
       None
     }
-  ui.foreach(_.bind())
 
   /** A default Hadoop Configuration for the Hadoop code (e.g. file systems) that we reuse. */
   val hadoopConfiguration = SparkHadoopUtil.get.newConfiguration(conf)
@@ -342,6 +341,10 @@ class SparkContext(config: SparkConf) extends Logging {
   postEnvironmentUpdate()
   postApplicationStart()
 
+  // Bind the SparkUI after starting the task scheduler
+  // because certain pages and listeners depend on it
+  ui.foreach(_.bind())
+
   private[spark] var checkpointDir: Option[String] = None
 
   // Thread Local variable that can be used by users to pass information down the stack