Text.style.textColor is undefined
With this file, which is reported by my plugin user:
[text_color_undefined.sketch.zip](https://github.com/sketch-hq/SketchAPI/files/4496597/text_color_undefined.sketch.zip)

executing following in playground:

```ts
let document = sketch.getSelectedDocument();
let layer = document.pages[0].layers[0].layers[0];
console.log(
    layer.name,
    layer.style.textColor, //undefined
    layer.style.fontFamily, //undefined
)
```

But the Text does have color and font family setting.

PS: I don't know how or in what version this Text is created. I just copied it from user's file.

 PS: More information from` json`, the `textStyle` is missing. But should the api handle this correctly?

<img width="1046" alt="image" src="https://user-images.githubusercontent.com/16953333/79638492-23a1e700-81b8-11ea-9ab3-c99fdcd27d03.png">
