Google map error: a is null

You are probably not listening for the onload event that fires when the page is completely loaded. As a result, your script is running but the div you are creating doesn't yet exist. Use jQuery to listen for this event, like so:

$(document).ready(function () {
    map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);
});

If you don't want to use jQuery, then add an event listener to body.onload

Make sure you specify the size of the element that holds the map. For example:

<div id="map_canvas" style="width: 500px; height: 500px;"></div>

Also make sure your map variable is defined in the global scope and that you initialize the map once the DOM is loaded.