@Override
      public void close() {
        synchronized (ClientHealthMonitoringRegion.class) {
          ClientHealthMonitoringRegion.currentInstance = null;
        }
      }
This method prepares a CacheListener, responsible for the cleanup of reference of admin region,
upon the cache closure.

@return CacheListener.
geode-core/src/main/java/org/apache/geode/internal/admin/ClientHealthMonitoringRegion.java