constructors return (null) without parent param
```
var text = new sketch.Text({
  text: 'my text',
  alignment: sketch.Text.Alignment.center,
})
```
returns `(null)` 

```
var text = new sketch.Text({
  parent: page,
  text: 'my text',
  alignment: sketch.Text.Alignment.center,
})
```
returns the layer and inserts it into a page.

```
var text = new sketch.Text({
  text: 'my text',
  alignment: sketch.Text.Alignment.center,
})
page.layers = [text];
```
works fine, so the layer is clearly being constructed OK behind-the-scenes.

I think the `(null)` printing is the misleading bit, as the top example is the constructor example from the API and so I thought it was broken when it was printing `(null)`

tested happening on `Text`, `Group` and `Shape`, so I assume all `Layer` ones