SymbolInstance.master throws error when wrapping MSImmutableSymbolInstance
The base problem isn't really a Sketch API issue, but in Sketch 53.1, a SymbolInstance object wrapping an MSImmutableSymbolInstance (for example, accessed from the document in the ExportSlices action) throws the following error when attempting to read the `master` property:
```
TypeError: this._object.symbolMaster is not a function. (In 'this._object.symbolMaster()', 'this._object.symbolMaster' is undefined)
```
I had a similar issue in Sketch 52 with Text layers (MSImmutableTextLayer didn't implement everything), and for this had to update my workaround that ensured that I got a mutable version of the current document - currently using `NSDocumentController.sharedDocumentController().currentDocument()`