#vulnerable code 
                             private List<? extends Number> getNodeAttribute(String key){
		NDArrayWrapper nodes = (NDArrayWrapper)get("nodes");

		Map<String, ?> content = (Map<String, ?>)nodes.getContent();

		return (List<? extends Number>)content.get(key);
	} 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             private List<? extends Number> getNodeAttribute(String key){
		List<? extends Number> nodeAttributes = (List<? extends Number>)ClassDictUtil.getArray(this, "nodes", key);

		return nodeAttributes;
	}