Can't iterate over selected layers with selectedLayers
I follow the guide on this [page](http://developer.sketchapp.com/reference/api/), however the code below doesn't work

``` js
var sketch = context.api()
var document = sketch.selectedDocument
var selection = document.selectedLayers
selection.iterate( function (item) { log( item.name() ) } )
```

It would throw this error:

```
TypeError: undefined is not an object (evaluating 'this._object._application.wrapObject')
line: 1627
sourceURL: /Applications/Sketch.app/Contents/Resources/SketchAPI.js
column: 54
```

But this works as expected：

``` js
var selection = context.selection

for (var i = 0; i < selection.length; i++) {
  var _selection = selection[i]
  log(_selection.name())
}
```

And here is my env:

``` js
log(sketch.api_version) // 1.1
log(sketch.version) // 40.2
log(sketch.build) // 33826
```
