Chrome extension options page 'addEventListener' bug

I'm trying to make an options page for my chrome extension. I've followed this tutorial exactly: https://developer.chrome.com/extensions/options.html

Now it doesn't do anything when pressing the 'save' button. I inspected the code and it returns Uncaught TypeError: Cannot call method 'addEventListener' of null
at the line: document.querySelector('#save').addEventListener('click', save_options);