null checks

Author: Minni Mittal

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/ApplicationMasterService.java

@@ -24,6 +24,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 
@@ -498,12 +499,17 @@ protected RMAppAttemptMetrics getAppAttemptMetrics(
     if (app == null) {
       return null;
     }
-    RMAppAttempt attempt = app.getAppAttempts().get(appAttemptId);
-    if (attempt == null) {
+
+    Map<ApplicationAttemptId, RMAppAttempt> attempts = app.getAppAttempts();
+    if (attempts == null) {
       return null;
+    } else {
+      RMAppAttempt attempt = attempts.get(appAttemptId);
+      if (attempt == null) {
+        return null;
+      }
+      return attempt.getRMAppAttemptMetrics();
     }
-
-    return attempt.getRMAppAttemptMetrics();
   }
 
   public void registerAppAttempt(ApplicationAttemptId attemptId) {