Get component JSON info through Sketch Api
I try to get Page JSON info through Sketch api like this:
```
export const exportCode = (context) => {
  var document = sketch.getSelectedDocument();
  const wrapPage = sketch.fromNative(document.selectedPage);
  console.log(wrapPage.toJSON());
};
```
I got an error:
![b429c625-70df-4520-986a-8810ccb7901e](https://user-images.githubusercontent.com/11405052/48064162-06669c00-e202-11e8-80a9-4a2a7d0650a0.png)


