System.getDeviceSettings().uniqueIdentifier not found

I have recently switch to developing on Visual Studio and my previously working app is experiencing:

Error: Symbol Not Found Error Details: Could not find symbol 'uniqueIdentifier'

while executing:

var mySettings = System.getDeviceSettings();
        var id = mySettings.uniqueIdentifier;
        if (id != null) {
            System.println(id); //e.g. ac915d426451c88e8ea691fa412f9af9c21b4d12
        }
Running in the emulator with SDK version 4.0.6 on a Fenix 6S. 
Has uniqueIdentifier been deprecated? This was working with SDK 4 previously.