@Test(expected = CoprocessorException.class)
  public void testPreFlushReturningNull() throws IOException {
    RegionCoprocessorHost rch = getRegionCoprocessorHost();
    rch.preFlush(null, null, null);
  }
Ensure we get expected exception when we try to return null from a preFlush call.
@throws IOException We expect it to throw {@link CoprocessorException}
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestRegionObserverPreFlushAndPreCompact.java