can't import background-image
I can't import a background-image.
Is my environment a problem?

macOS : 10.12.6
sketch : 50.2

**Sketch dev tool message**
![2018-06-26 10 30 26](https://user-images.githubusercontent.com/16788797/41915775-7fb2e1d8-7991-11e8-9b27-dafa5f6491b2.png)

**sketch result**
![2018-06-26 10 36 32](https://user-images.githubusercontent.com/16788797/41915778-80dfc01c-7991-11e8-8abd-d6483ebc5084.png)

**valid json file from `npm run e2e-debug`**
[backgrounds.html.asketch.json.zip](https://github.com/brainly/html-sketchapp/files/2137366/backgrounds.html.asketch.json.zip)