karthikhw opened a new pull request #876: HBASE-23338 Prevent NPE running 
rsgroup balancer
URL: https://github.com/apache/hbase/pull/876
 
 
   I think we should prevent NPE even if user trigger balancer on rsgroup that 
contains only a single regionserver.  Sometimes users trigger balancer on all 
groups even without knowing group info.
   
   master log
   ```
   2019-11-24
   22:50:29,868 ERROR 
[RpcServer.default.FPBQ.Fifo.handler=22,queue=1,port=16000]
   ipc.RpcServer: Unexpected throwable object
   java.lang.NullPointerException
   ```
   
   Shell
   ```
   hbase(main):002:0> balance_rsgroup 'shva'
   ERROR: java.io.IOException at 
org.apache.hadoop.hbase.ipc.RpcServer.call(RpcServer.java:433) at 
org.apache.hadoop.hbase.ipc.CallRunner.run(CallRunner.java:133) at 
org.apache.hadoop.hbase.ipc.RpcExecutor$Handler.run(RpcExecutor.java:338) at 
org.apache.hadoop.hbase.ipc.RpcExecutor$Handler.run(RpcExecutor.java:318)Caused 
by: java.lang.NullPointerException
   For usage try 'help "balance_rsgroup"'
   ```

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