centerOnLayer not centering nested layers
Currently i am working on a plugin that scans your Sketch for all colors you have used in the document and list them for you per color. In the tree structure we list all the layers, and clicking them selects them and does a `centerOnLayer` in sketch.

This works fine for layers that are not nested:
<img width="1680" alt="Screenshot 2019-04-18 at 10 01 40" src="https://user-images.githubusercontent.com/22663593/56345903-1f1baa80-61c1-11e9-9935-b1887f8a7755.png">
<img width="1680" alt="Screenshot 2019-04-18 at 10 02 14" src="https://user-images.githubusercontent.com/22663593/56345904-1f1baa80-61c1-11e9-9ff6-9fe13a7df1cd.png">

But for the layers that are in the group, the nested layers, the `centerOnLayer` does not center the layers but puts the off to the bottom right as you can see here:
<img width="1680" alt="Screenshot 2019-04-18 at 10 02 33" src="https://user-images.githubusercontent.com/22663593/56346007-63a74600-61c1-11e9-9c46-8d33256e53c7.png">

Here is the function that is being called from the webview:
``` javascript
  webContents.on('selectLayer', (layerID) => {
    const document = sketch.getDocuments()[0];
    const sketchLayer = document.getLayerWithID(layerID);
    document.centerOnLayer(sketchLayer);
    document.selectedLayers = [sketchLayer];
  });
```

It feels to me like it's an issue with the nesting/centerOnLayer, but maybe i am doing something wrong. Any help would be much appreciated!