NPE while running balance_rsgroup if any split region is present.

 NPE is thrown in below scenario:
1: Create rsgroup pgroup
2: Add two RS to pgroup
3: create two table with 15 region each
4: kill 1 RS and start again
5: disable t1
6: run balancer and balance_rsgroup 'pgroup' and observe it is working fine
7: now split one of the region of t2
8: run balance_rsgroup 'pgroup' and observe NPE:

ERROR: java.io.IOException
at org.apache.hadoop.hbase.ipc.RpcServer.call(RpcServer.java:517)
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
at java.util.TreeMap.put(TreeMap.java:563)
at org.apache.hadoop.hbase.rsgroup.RSGroupBasedLoadBalancer.correctAssignments(RSGroupBasedLoadBalancer.java:347)
at org.apache.hadoop.hbase.rsgroup.RSGroupBasedLoadBalancer.balanceCluster(RSGroupBasedLoadBalancer.java:140)
at org.apache.hadoop.hbase.rsgroup.RSGroupAdminServer.balanceRSGroup(RSGroupAdminServer.java:531)
at org.apache.hadoop.hbase.rsgroup.RSGroupAdminEndpoint$RSGroupAdminServiceImpl.balanceRSGroup(RSGroupAdminEndpoint.java:302)
at org.apache.hadoop.hbase.protobuf.generated.RSGroupAdminProtos$RSGroupAdminService.callMethod(RSGroupAdminProtos.java:16306)
at org.apache.hadoop.hbase.master.MasterRpcServices.execMasterService(MasterRpcServices.java:1023)
at org.apache.hadoop.hbase.shaded.protobuf.generated.MasterProtos$MasterService$2.callBlockingMethod(MasterProtos.java)
at org.apache.hadoop.hbase.ipc.RpcServer.call(RpcServer.java:458)