Sketch quit unexpectedly when bad JSON provided
**Describe the bug**
Sketch quit unexpectedly when bad JSON provided. I use this tool (https://www.csvjson.com/csv2json ) for generating JSON file with czech characters.

**To Reproduce**
Steps to reproduce the behavior:
1. Click on Populate witch JSON
2. Open this file [csvjson.json.zip](https://github.com/preciousforever/data-populator/files/2386009/csvjson.json.zip)
3. Click populate
4. See error

**Expected behavior**
Some error would be fine. It's quite hardcore to kill whole app.

**Environment (please complete the following information):**
 - macOS Version: macOS High Sierra 10.13.5
 - Sketch Version 51.3
 - Plugin Version 2.3.4
