Text.Style.fontAxes issue
[error.zip](https://github.com/sketch-hq/SketchAPI/files/5472494/error.zip)

Above is a file reported by one my user, many of them encountered issues in this part.

## How to reproduce

1. Sketch prompts font is missing
1. Replace the font with Work Sans-Medium

then, try:

```ts
    console.log(layer.style.fontAxes)
    console.log(JSON.stringify(layer.style.fontAxes))
    console.log(typeof (layer.style.fontAxes as any).Weight.value)
```

Output:

```
  { Weight: { id: 2003265652, min: 100, max: 900, value: 500 } }
  '{"Weight":{"id":{},"min":{},"max":{},"value":{}}}'
  'object'
```

## Issues

1. **If I assign above `layer.style` to another layer, Sketch crashes instantly**
1. `Text.Style.fontAxes` doesn't have property `Weight` according to documentation
2. All values provided by Sketch API should be JS type. But `Weight.value` is object (Obj-C?). So `JSON.stringify` doesn't work with it

No matter what has happened, I think the API should do the compatible magic to these circumstances.