I have a @Model class like so:

@Model
final class MyModel {
    
    @Attribute(.unique) var id: String
    var firestoreStruct: FirestoreStruct
    
    init(id: String, firestoreStruct: FirestoreStruct) {
        self.id = id
        self.firestoreStruct = firestoreStruct
    }
}

FirestoreStruct is a struct with which I fetch data from Firestore:

struct FirestoreStruct: Codable {
    
    @DocumentID var id: String?
    var title: String
}

My use case is to save the FirestoreStructs to fetch them from Widgets etc (but I don’t even get there).
When I try to insert and save an instance of MyModel to the modelContext, I get an error

Fatal error: ‘try!’ expression unexpectedly raised an error:
FirebaseFirestore.FirestoreEncodingError.encodingIsNotSupported(“DocumentID
values can only be encoded with Firestore.Encoder”)

This is the code saving the data. However the error only gets produced when saving. When I remove the second line of code, it does postpone the crash to when minimizing the app (I assume SwiftData then tries to save it).

modelContext.insert(MyModel(id: "this is the ID", firestoreStruct: FirestoreStruct(title: "title")))
try modelContext.save()

I’ve set up the modelContainer correctly (as I believe)

.modelContainer(for: [MyModel.self])

How can I avoid this error?