Error trying to export PNGs

### How did it happen?
Try to export PNG images for the artboards.

I'm getting this error on both a pre-existing file and a new, simple file. No PNGs get exported as a result of the error.

I've tried uninstalling all instances of `Git.sketchplugin` from my machine and starting over with a fresh new download of the plugin. Getting the same error.

On a larger file that I tried the export process on, the beach-ball cursor hung indefinitely. During a normal export process (which doesn't happen now), the cursor turns into the beach-ball for a couple minutes but then completes the export.

### Version info
Sketch version: 64
Git-Sketch-Plugin version: 0.12.0 (the new release as of 5 days ago).

### Error log

```
JSON Parse error: Unrecognized token ''

{
"line": 1,
"column": 32249,
"sourceURL": "/Users/jasonfrank/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Git.sketchplugin/Contents/Sketch/__commands_Export.js"
}

parse@[native code]
v@/Users/jasonfrank/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Git.sketchplugin/Contents/Sketch/__commands_Export.js:1:32249
/Users/jasonfrank/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Git.sketchplugin/Contents/Sketch/__commands_Export.js:1:34592
/Users/jasonfrank/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Git.sketchplugin/Contents/Sketch/__commands_Export.js:1:34545
/Users/jasonfrank/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Git.sketchplugin/Contents/Sketch/__commands_Export.js:1:34578
__skpm_run@/Users/jasonfrank/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Git.sketchplugin/Contents/Sketch/__commands_Export.js:1:35021
__skpm_run@[native code]
```

Reproduce PNG export in Sketch 64 with Git-Sketch-Plugin 0.12.0 using both a new simple file and a pre-existing file. Start with the JSON parse error and the generated Contents/Sketch/__commands_Export.js location shown in the log. Done means PNG export completes without the parse error or an indefinite beach-ball hang.

Written by the indexing model from the issue text.