Error when adding a file to git
### How did it happen?
1. I opened my (edited) material-baseline.sketch
2. I opened menu Plugins > Git > Advanced > Add file to git
3. Then the crash happened


### Error log

```
{
	"nativeException": {},
	"line": 195,
	"column": 37,
	"sourceURL": "/Users/remco.tkp/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Git.sketchplugin/Contents/Sketch/Add.js"
}
```