YARN-9507. Fix NPE in NodeManager#serviceStop on startup failure. Contributed by Bilwa S T.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-nodemanager/src/main/java/org/apache/hadoop/yarn/server/nodemanager/NodeManager.java

@@ -526,9 +526,11 @@ public class NodeManager extends CompositeService
       DefaultMetricsSystem.shutdown();
 
       // Cleanup ResourcePluginManager
-      ResourcePluginManager rpm = context.getResourcePluginManager();
-      if (rpm != null) {
-        rpm.cleanup();
+      if (null != context) {
+        ResourcePluginManager rpm = context.getResourcePluginManager();
+        if (rpm != null) {
+          rpm.cleanup();
+        }
       }
     } finally {
       // YARN-3641: NM's services stop get failed shouldn't block the