Assertion Failed: calling set on destroyed object in FocusableMixin#handleMouseLeave

This only happens in Safari, as far as I can tell.
```
[Error] Error: Assertion Failed: calling set on destroyed object: .hover = false
assert (vendor.js:52288)
set (vendor.js:31232)
handleMouseLeave (vendor.js:199445)
superWrapper (vendor.js:47031)
superWrapper
```

`handleMouseLeave` refers to https://github.com/miguelcobain/ember-paper/blob/master/addon/mixins/focusable-mixin.js#L85

The `PaperButton` in question is of the form
```
"}} />
```

so nothing fancy.

This error happens after the transition to the new route occurs, and the user moves the mouse.