YARN-7508. NPE in FiCaSchedulerApp when debug log enabled in async-scheduling mode. Contributed by Tao Yang.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/common/fica/FiCaSchedulerApp.java

@@ -439,9 +439,9 @@ public boolean accept(Resource cluster,
               if (LOG.isDebugEnabled()) {
                 LOG.debug("Try to re-reserve a container, but node "
                     + schedulerContainer.getSchedulerNode()
-                    + " is already reserved by another container"
+                    + " is already reserved by another container="
                     + schedulerContainer.getSchedulerNode()
-                    .getReservedContainer().getContainerId());
+                    .getReservedContainer());
               }
               return false;
             }