Throw an error when document.activeElement was null

[hidekatsu-izuno]

I use both sapper and bable in combination for the compatibilty.
Then I saw the error that be unable to get property 'blur' of undefined or null reference.

document.activeElement.blur();`

This problem may only occur on IE. (I tested on IE11). When any elements are not focused, document.activeElement return document.body on the other browser. But there is a posibility that a element will be null or non element on IE.

So, it may have to rewrite the below a code.

try {
    document.activeElement.blur();
} catch (e) {
    document.body.blur()
}

[Rich-Harris]

thanks! this is fixed in 0.15.5