Uncaught TypeError: Cannot set property 'onchange' of null

Question

I am using a drop down for dynamically changing content on a page. It works but is throwing a js error in Chrome ONLY. What Chrome recommends I don't know how to implement. Site is built in ExpressionEngine 2.8.1.

ERROR MESSAGE IN CHROME

Uncaught TypeError: Cannot set property 'onchange' of null functions.js:65
event.returnValue is deprecated. Please use the standard event.preventDefault() instead.

MY JS Code

document.getElementById("drop").onchange = function() {
    if (this.selectedIndex!==0) {
        window.location.href = this.value;
    }        
};

MY HTML Code

            <form method="post" action="{path='locations/index'}" class="drop">
                <select id="drop">
                    <option>Select a Location:</option>
                {exp:channel:entries channel="locations" category="not 3" orderby="title" sort="asc" dynamic="no"}
                    <option value="{site_url}index.php/locations/{url_title}">{title}</option>
                {/exp:channel:entries}
                </select>
            </form>

Answer 1

There's only one possible reason for that error message, document.getElementById("drop") does not return the element, and the only reason for that is that the element doesn't exists, but in the HTML it clearly does, so the script must be running before the elements in the DOM.

You have to include the javascript after the elements in the DOM, or wrap it in a DOM ready handler, like window.onload etc.

<form method="post" action="{path='locations/index'}" class="drop">
    <select id="drop">
        <option>Select a Location:</option>{exp:channel:entries channel="locations" category="not 3" orderby="title" sort="asc" dynamic="no"}
        <option value="{site_url}index.php/locations/{url_title}">{title}</option>{/exp:channel:entries}
    </select>
</form>
<script>
    document.getElementById("drop").onchange = function() {
        if (this.selectedIndex !== 0) {
            window.location.href = this.value;
        }
    };
</script>