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URLから天気情報を取得するJavaScriptを書いています。しかし、Google Chrome には次のエラーが表示されます。

Uncaught TypeError: Cannot call method 'addEventListener' of null weather.html:37
(anonymous function)

これを修正するための提案はありますか? ありがとう！

<html>
<head>
    <title>My weather</title>
    <script type="text/javascript">
    function fetchWeather(){
        var xmlHttp = new XMLHttpRequest(); // Initialize our XMLHttpRequest instance
        xmlHttp.open("GET", "http://classes.engineering.wustl.edu/cse330/content/weather_json.php", true);
        xmlHttp.addEventListener("load", ajaxCallback, false);
        xmlHttp.send(null);

    }

    function ajaxCallback(event){
        var htmlParent = document.getElementById("weatherWidget"); // Get the HTML element
        var jsonData = JSON.parse(event.target.responseText);

        var loc = document.getElementById("weatherWidget").getElementsByClassName("weather-loc")[0];
        loc.appendChild(document.createTextNode(jsonData.location.city+" "+jsonData.location.state));

        var humidity = document.getElementById("weatherWidget").getElementsByClassName("weather-humidity")[0];
        humidity.appendChild(document.createTextNode("Humidity: "+jsonData.atmosphere.humidity+"%"));

        var temp = document.getElementById("weatherWidget").getElementsByClassName("weather-temp")[0];
        temp.appendChild(document.createTextNode("Currently: "+jsonData.current.temp));

        var tomorrow = document.getElementById("weatherWidget").getElementsByClassName("weather-tomorrow")[0];
        tomorrow.appendChild(document.setAttribute("src", jsonData.current.temp));
        tomorrow.alt = jsonData.tomorrow.text;
        tomorrow.src = "http://us.yimg.com/i/us/nws/weather/gr/"+jsonData.tomorrow.code+"ds.png" 
        var dayaftertomorrow = document.getElementById("weatherWidget").getElementsByClassName("weather-dayaftertomorrow")[0];
        dayaftertomorrow.appendChild(document.setAttribute("src", jsonData.current.temp));
        dayaftertomorrow.alt = jsonData.dayafter.text;
        dayaftertomorrow.src = "http://us.yimg.com/i/us/nws/weather/gr/"+jsonData.dayafter.code+"ds.png"

    }

    document.getElementById("update").addEventListener("click", fetchWeather, false); // bind fetchWeather() to the DOMContentLoaded event 
                                                                                      //so that your weather widget is automatically initialized 
                                                                                      //when the page is loaded
    </script>
</head>
<body>

<div class="weather" id="weatherWidget">
    <div class="weather-loc"></div>
    <div class="weather-humidity"></div>
    <div class="weather-temp"></div>
    <img class="weather-tomorrow" />
    <img class="weather-dayaftertomorrow" />
</div>
<button id="update">Update</button>

</body>
</html>
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