export foreign symbols
Is there any way to export a group layer in foreign symbols to buffer?
I try this and get an error.
```js
let symbolReferences = library.getImportableSymbolReferencesForDocument(document);
symbolReferences.forEach(importableSymbol => {
        let symbolReference = importableSymbol.import();
        let layers = symbolReference.layers;
        let options = { formats: 'svg', output: false };
        for (let i = 0; i < layers.length; i++) {
             if (layers[i].type === 'Group') {
                    var duplicatedLayer = layers[i].duplicate();
                    let buffer = sketch.export(duplicatedLayer, options);
                    break;
             }
         }
})
```
When the group is in current document, not in library, I can export it to svg successfully.