Fix bug on back button where animation isn't found

[clw8]

Clicking the back button on Chrome gives the following error:

For some reason, when going back the animation isn't found, so the modified code is just to check whether the animation exists before executing the code

[jaxtheking]

I ended up experiencing the same issue after I set animateHistoryBrowsing to false. To reproduce it:

• Navigate through few pages
• Reload the page you're on
• Now click the browser's back button: you'll see the error above in the console.

Any chance this fix could be reviewed so that no error is raised?

[daun]

Thanks for your contribution! I'll close this in favor of #18 which solves this in a slightly different way.