#vulnerable code 
                             private void refreshTreeData() {
		Collection<ZkCluster> zkClusters = RegistryCenterServiceImpl.ZKADDR_TO_ZKCLUSTER_MAP.values();
		for (ZkCluster zkCluster : zkClusters) {
			InitRegistryCenterService.initTreeJson(REGISTRY_CENTER_CONFIGURATION_MAP.get(zkCluster.getZkAddr()), zkCluster.getZkAddr());
		}
	} 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             private void refreshTreeData() {
		Collection<ZkCluster> zkClusters = RegistryCenterServiceImpl.ZKADDR_TO_ZKCLUSTER_MAP.values();
		for (ZkCluster zkCluster : zkClusters) {
			InitRegistryCenterService.initTreeJson(zkCluster.getRegCenterConfList(), zkCluster.getZkAddr());
		}
	}