Freezes when JSON contains Emoji
**Describe the bug**
Freezes when JSON contains Emoji.

**To Reproduce**
Steps to reproduce the behavior:
1. Create JSON with any Emoji in it
2. Try to populate symbol
3. Freeze


**Environment (please complete the following information):**
 - macOS Version: 10.13.1
 - Sketch Version 48.2
 - Plugin Version 2.3.4

