Uncaught TypeError: Cannot read property 'style' of null

[seeker09-0]

Error on Google Chrome 49.0.2601.0 canary (64-bit)

Error on Firefox 43.0.3

I'm using the script in a Drupal theme (by loading on the theme_name.info file) that includes it in the html head.
Probably it should only run after the page is loaded.

I've tried to comment the stylefill.init in the mm-fontsize.js and call it on window.onload, and the error was gone but there were no visible effects (the new css rules were not being applied)