Review the javascript error your browser produces when the button is
poked
'map' is undefined.
Your 'map' is only defined within your initialize() function, and
after the function has completed it is no longer available to other
functions (such as your button)
function initialize() {
...
var map = new google.maps....
Define your empty 'map' variable in global scope, and change
initialize() to create your API map using the global 'map' variable.
var map;
function initialize() {
...
map = new google.maps....
It's basic javascript, not maps magic