[SPARK-29046] Fix NPE in SQLConf.get when active SparkContext is stopping

Author: HeartSaVioR

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -139,7 +139,8 @@ object SQLConf {
       }
     } else {
       val isSchedulerEventLoopThread = SparkContext.getActive
-        .map(_.dagScheduler.eventProcessLoop.eventThread)
+        .flatMap { sc => Option(sc.dagScheduler) }
+        .map(_.eventProcessLoop.eventThread)
         .exists(_.getId == Thread.currentThread().getId)
       if (isSchedulerEventLoopThread) {
         // DAGScheduler event loop thread does not have an active SparkSession, the `confGetter`