Native.js 怎么调用ios的的PHAsset库？

ios获取最新一张照片

        PHFetchOptions * options = [[PHFetchOptions alloc] init];  
        options.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey: @ "creationDate" ascending: NO]];  
        PHFetchResult * assetsFetchResults = [PHAsset fetchAssetsWithOptions: options];  
        return [assetsFetchResults firstObject];

我写的 Native.js，似乎有什么不对……

        let Photos = plus.ios.importClass("Photos");  

        let NSSortDescriptor = plus.ios.importClass("NSSortDescriptor");  

        let sort = new NSSortDescriptor();  
        plus.ios.invoke(sort, "sortDescriptorWithKey:ascending:", "creationDate", false);  

        let PHFetchOptions = plus.ios.importClass("PHFetchOptions");  

        let options = new PHFetchOptions();  
        options.sortDescriptors = sort;  

        let PHAsset = plus.ios.importClass("PHAsset");  
        let asset = new PHAsset();  

        let PHFetchResult = plus.ios.importClass("PHFetchResult");  
        let assetsPHFetchResult = new PHFetchResult();  

        assetsPHFetchResult = plus.ios.invoke(asset, "fetchAssetsWithOptions:", options);  
        assetsPHFetchResult.firstObject()

错误提示：undefined is not an object (evaluating 'r.firstObject')