Cannot build with Parcel: Unterminated regular expression

```
$ PRODUCT_TYPE=webapp parcel entry/src/index.html --open
Server running at http://localhost:1234
⚠️ Could not load existing sourcemap of "../../node_modules/react-spreadsheet-grid/lib/bundle.js".
🚨 /node_modules/react-spreadsheet-grid/lib/bundle.js:520:83: Unterminated regular expression (520:83)
518 | if (sourceMap) {
519 | // http://stackoverflow.com/a/26603875
> 520 | css += "\n + btoa(unescape(encodeURIComponent(JSON.stringify(sourceMap)))) + " */";
| ^
521 | }
522 |
523 | var blob = new Blob([css], { type: "text/css" });
```

* `react-spreadsheet-grid` version: 1.4.3
* `parcel-bundler` version: 1.12.4