第1行第1行的PHP错误：文档为空

Question

尝试运行以下代码时出现错误error on line 1 at column 1: Document is empty。可能导致什么呢？

function Twitter_RSS() {
  return; 
}

function doGet(e) {

  var widgetID = e.queryString? e.queryString : "ERROR_NO_ID_FOUND";
  var cache = CacheService.getPublicCache();
  var id = "Twitter" + widgetID;
  var rss = cache.get(id);

  if ( ! rss ) {
    rss = getTweets(widgetID);
    cache.put(id, rss, 120); // Expire in 2 minutes
  }

  return ContentService.createTextOutput(rss)
  .setMimeType(ContentService.MimeType.XML);
}


function getTweets(id) {

  try {

    var widget, json, tweets, regex, tweet, list, time, url, when, rss, heading, title, link, alltweets, permalink, permatitle; 

    title = "Twitter RSS Feed : " + id;
    link  = "http://www.techcovered.org/#" + id;              
    url   = "http://cdn.syndication.twimg.com/widgets/timelines/" + id;

    widget  = UrlFetchApp.fetch(url);
    json    = Utilities.jsonParse(widget);   

    if ( ! json.body ) {
      return;
    }

    list = json.body.replace(/(\r\n|\n|\r)/gm," ")
                    .replace(/\s+/g, " ")
                    .replace(/<div class=\"(h-card|footer|detail-expander|retweet-credit)[^>]*>(.*?)<\/div>/gi, "")
                    .replace(/<time[^>]*>(.*?)<\/time>/gi, "");

    regex = new RegExp(/<h1[^>]*>(.*?)<\/h1>/ig);

    if ((heading = regex.exec(list)) !== null) {

      regex = RegExp(/href="(.*?)"/ig);
      if ((permalink = regex.exec(heading[1])) !== null) {
        link = permalink[1];
      }

      regex = RegExp(/title="(.*?)"/ig);
      if ((permatitle = regex.exec(heading[1])) !== null) {
        title = permatitle[1];
      }
    }

    var self = ScriptApp.getService().getUrl() + "?" + id;

    rss  = '<?xml version="1.0"?><rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">';
    rss += ' <channel><title>' + title + '</title>';
    rss += ' <link>' + link + '</link>';
    rss += ' <atom:link href="' + self + '" rel="self" type="application/rss+xml" />';
    rss += ' <description>' + title + ' :: RSS Generator available at http://www.labnol.org/?p=28149</description>';

    regex = RegExp(/<ol[^>]*>(.*?)<\/ol>/ig);

    if ((alltweets = regex.exec(list)) !== null) {

      alltweets = alltweets[1].replace(/\s+/g, " ");

      var re = /<a class=".*?permalink.*?" href="(.*?)" data-datetime="(.*?)"[^>]*>(.*?)<\/a>(.*?)<p[^>]*>(.*?)<\/p>/gm;

      while (tweet = re.exec(alltweets)) {

        url   = tweet[1];
        when  = Utilities.formatDate(parseDate(tweet[2]), "UTC", "EEE, d MMM yyyy HH:mm:ss");

        tweet = tweet[5].replace(/<\s*(div|span|b|p)[^>]*>/gi, "")
                        .replace(/<\s*\/\s*(div|span|b|p)[^>]*>/gi, "")
                        .replace(/class=".*?"|rel=".*?"|title=".*?"|target=".*?"|data-expanded-url=".*?"|data-query-source=".*?"|dir=".*?"|data-scribe=".*?"/gi, "")
                        .replace(/\s+/g, " ");
        if(tweet.substring(0,2)!="RT") {
        rss += "<item>";
        rss += " <title>"+ tweet.replace(/<a[^>]*>(.*?)<\/a>/gi, "") + "</title>";
        rss += " <pubDate>" + when + " +0000</pubDate>";
        rss += " <guid>" + url + "</guid>";
        rss += " <link>" + url + "</link>";
        rss += " <description><![CDATA[ @"+url.split("/")[3] + ": " + tweet + "]]></description>";
        rss += "</item>";            
        } 

      }

      rss += "</channel></rss>";
      return rss;
    }

  } catch (e) {
    Logger.log(e.toString());
  }
}

function parseDate(d) {

  var date = /(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})/;
  var m = date.exec(d);
  var year   = +m[1];
  var month  = +m[2];
  var day    = +m[3];
  var hour   = +m[4];
  var minute = +m[5];
  var second = +m[6];

  return new Date(year, month - 1, day, hour, minute, second);
}