Swift loading JSON in extension of struct - swift

Question

Given this (partially pasted) struct:
struct InstrumentsSet: Identifiable, Decodable {
static func withJSON(_ fileName: String) -> InstrumentsSet? {
guard let url = Bundle.main.url(forResource: fileName, withExtension: "json", subdirectory: "Sets") else { return nil }
guard let data = try? Data(contentsOf: url) else { return nil }
return try? JSONDecoder().decode(InstrumentsSet.self, from: data)
}
let name: String
let bpm: Double
let timeSignature: Int
var tracks: [Track]
}
And this extension:
extension InstrumentsSet {
struct Track: Identifiable, Decodable {
static func withJSON(_ fileName: String) -> InstrumentsSet.Track? {
guard let url = Bundle.main.url(forResource: fileName, withExtension: "json", subdirectory: "Sets") else { return nil }
guard let data = try? Data(contentsOf: url) else { return nil }
//At this point data has the JSON loaded and decode makes it jump to the fatal error located at the call
return try? JSONDecoder().decode(InstrumentsSet.Track.self, from: data)
}
let instrumentType: InstrumentType
let startType: StartType
}
}
I can load successfully an instrument set from JSON with nested Tracks this way:
guard var instrumentSet1 = InstrumentsSet.withJSON("file-with-set") else {
fatalError("Error loading set JSON")
}
Next I'd like to add a track to the existing set by loading a json with just Track data:
guard let masterTrack = InstrumentsSet.Track.withJSON("file-with-track") else {
fatalError("Error loading track JSON")
}
When running this part I can see the JSON data loading successfully, but on this line return try? JSONDecoder().decode(InstrumentsSet.Track.self, from: data) the fatalError is triggered without further explanation. My guess is something is missing in the instrumentSet part since the function "InstrumentsSet.Track.withJSON" is called through the original InstrumentsSet struct so passing down just a track is not possible?
My question, given the struct and its extension, is it possible to load just the extension part (Track) of the struct into the let masterTrack?
Example Set JSON:
{
"name": "Set name",
"bpm": 124.00,
"timeSignature": 4,
"tracks": [
{
"instrumentType": "exsSampler",
"startType": "trigger"
}
]
}
Example Track JSON. (I've tried passing this as an Array, but still no success)
{
"instrumentType": "audioBuffer",
"startType": "global"
}
Thank you!