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我正在为 RSS 新闻提要构建一个 AJAX 加载器。我遇到了一个基本的 JS 错误，但我不明白为什么。这是一个未捕获的类型错误：无法读取 null 的选项属性。

这是我的代码，如果您在我的事件处理程序或我的 JS 中看到了什么，请告诉我。

HTML： 

<label for="feeds">Select A News Feed</label>
<select id="feeds" name="feeds">
    <option value="Yahoo">Get Yahoo News Top Stories</option>
    <option value="CNN">Get CNN Arabic Top News</option>
    <option value="BBC">Get BBC World Video and Audio</option>
    <option value="Figaro">Get Le Figaro Politique Stories</option>
</select>
<br/>
<label for="items">Number of Items Displayed:</label>
<select id="items" name="items">
    <option value="3">3</option>
    <option value="5">5</option>
    <option value="10">10</option>
    <option value="50">50</option>
    <option value="100">100</option>
</select>
<br/>
<button onclick="return readrss.doxhr('newsContainer1', readrss.feedUrl)" id="button">Get News</button>
<br/>
<div id="newsContainer1">
</div>

JS：

var itemSelect = document.getElementById("items");
var feedSelect = document.getElementById("feeds");

readrss = {

timeOutDuration: 2,

toolong: false,

doxhr: function (container, url) {
     readrss.numOfItems= itemSelect.options[itemSelect.selectedIndex].value; //get number of items to display

     readrss.feedUrl = feedSelect.options[feedSelect.selectedIndex].value; //get URL from selected value
    if (!document.getElementById || !document.createTextNode) {
        return;
    }
    readrss.outputContainer = document.getElementById(container);
    if (!readrss.outputContainer) {
        return;
    }
    var request;
    try {
        request = new XMLHttpRequest();
    } catch (error) {
        try {
            request = new ActiveXObject("Microsoft.XMLHTTP");
        } catch (error) {
            return true;
        }
    }
    request.onreadystatechange = function () {
        if (request.readyState == 1) {
            readrss.toolong = window.setTimeout(

            function () {
                if (request.readyState == 1) {
                    readrss.timedout = true;
                    request.abort();
                    readrss.outputContainer.innerHTML = 'The request took too long';
                }
            },
            readrss.timeOutDuration);
            readrss.outputContainer.innerHTML = '<img src = "indicator_medium.gif" alt = "loading" />';
        }

        if (request.readyState == 4 && !readrss.timedout) {
            window.clearTimeout(readrss.toolong);
            if (/200|304/.test(request.status)) {
                readrss.retrieved(request);
            } else {
                readrss.failed(request);
            }
        }
    };
    request.open('get', '../loadrss.php?url = ' + encodeURI(readrss.feedUrl));
    request.setRequestHeader('If-Modified-Since', 'Wed, 05 Apr 2006 00:00:00 GMT');
    request.send(null);
    return false;
},
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