It looks like
HDFS-12487 changes the error message expected by TestDiskBalancer#testDiskBalancerWithFedClusterWithOneNameServiceEmpty.
The test expects error "There are no blocks in the blockPool" but after
HDFS-12487, it returns error string "NextBlock call returned null.No valid block to copy."
Probably the simplest approach to fix it is to update the expected error string.