YARN-11560. Fix NPE bug when multi-node enabled with schedule asynchronously (#6021). Contributed by wangzhongwei.

Author: granewang

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/CapacityScheduler.java

@@ -1737,6 +1737,10 @@ private CSAssignment allocateContainerOnSingleNode(
 
   private void allocateFromReservedContainer(FiCaSchedulerNode node,
       boolean withNodeHeartbeat, RMContainer reservedContainer) {
+    if(reservedContainer == null){
+      LOG.warn("reservedContainer is null, that may be unreserved by the proposal judgment thread");
+      return;
+    }
     FiCaSchedulerApp reservedApplication = getCurrentAttemptForContainer(
         reservedContainer.getContainerId());
     if (reservedApplication == null) {