Prevent restoring focus to last element if none exist

[rhuynh]

Fixed a null pointer exception when trying to restore focus to the last element where that element doesn't exist.

[rhuynh]

There is a specific case where this bug exists in ie11. In my application, if you trigger the alertify dialog as a result of a button and you remove that button from the dom, when you click on the dialog ok button, you will get the exception as elCallee === null. This is due to document.activeElement will sometime be null.

[fabien-d]

thoughts on a fallback to reset the tabbing focus at the top of the page instead of the bottom?

if (elCallee) {
    elCallee.focus();
} else {
    document.body.focus();
}

[rhuynh]

i have thought about that option, but what i found is that the browser will show the focus border around the body and it does seem kinda strange/annoying since the user didn't trigger the alertrify from the body (i know that you can hide this w/ css, but it seems unnecessary). is it necessary to always reset the focus back to some element after the dialog is closed?

[fabien-d]

is it necessary to always reset the focus back to some element after the dialog is closed?

Nope. I was just curious!

If you could just squash your commits, I'll merge. Thanks