Can't save Updated ML Model - gives nil error

Code Block 
    private static func saveUpdatedModel(_ updateContext: MLUpdateContext) {
        let updatedModel = updateContext.model
        let fileManager = FileManager.default
        do {
            // Create a directory for the updated model.
            try fileManager.createDirectory(at: tempUpdatedModelURL,
                                            withIntermediateDirectories: true,
                                            attributes: nil)
            
            // Save the updated model to temporary filename.
            try updatedModel.write(to: tempUpdatedModelURL)
            print("tempUpdatedModelURL - \(tempUpdatedModelURL)")
            
            // Replace any previously updated model with this one.
            _ = try fileManager.replaceItemAt(updatedModelURL,
                                              withItemAt: tempUpdatedModelURL)
            
            print("Updated model saved to:\n\t\(updatedModelURL)")
        } catch let error {
            print("Could not save updated model to the file system: \(error)")
            return
        }
    }

Code Block 
extension UpdatableDrawingClassifier {
    /// Creates an update model from a given model URL and training data.
    ///
    /// - Parameters:
    ///     - url: The location of the model the Update Task will update.
    ///     - trainingData: The training data the Update Task uses to update the model.
    ///     - completionHandler: A closure the Update Task calls when it finishes updating the model.
    /// - Tag: CreateUpdateTask
    static func updateModel(at url: URL,
                            with trainingData: MLBatchProvider,
                            completionHandler: @escaping (MLUpdateContext) -> Void) {
        
        // Create an Update Task.
        guard let updateTask = try? MLUpdateTask(forModelAt: url,
                                           trainingData: trainingData,
                                           configuration: nil,
                                           completionHandler: completionHandler)
        
            else {
                print("Could't create an MLUpdateTask.")
                return
        }
        
        updateTask.resume()
        print("Updating Model - updateTask.resume()")
        print("URL - \(url)")
    }
}