YARN-9246 NPE when executing a command yarn node -status or -states without additional arguments. Contributed by Masahiro Tanaka

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-client/src/main/java/org/apache/hadoop/yarn/client/cli/YarnCLI.java

@@ -72,6 +72,10 @@ public void setClient(YarnClient client) {
   }
 
   public void stop() {
+    // this.client may be null when it is called before
+    // invoking `createAndStartYarnClient`
+    if (this.client != null) {
       this.client.stop();
     }
   }
+}