CreateML - problems with asyncronous training

I can successfully train an ActionClassifier using CreateML. However, I get crashes when I attempt to do the same asynchronously.

The model parameters and training data sources are the same in both cases:

let modelParameters = MLActionClassifier.ModelParameters(validation: validationDataSet,batchSize: 5, maximumIterations: 10, predictionWindowSize: 120, targetFrameRate: 30)

let trainingDataSource = MLActionClassifier.DataSource.directoryWithVideosAndAnnotation(at: myStudyParticipantURLFinal, annotationFile: documentURLFinal, videoColumn: "file", labelColumn: "category", startTimeColumn: "startTime", endTimeColumn: "endTime")

the only thing I add to attempt asyncrounous training is sessionParameters:

let sessionDirectory = URL(fileURLWithPath: "(NSHomeDirectory())/test")

                            // Session parameters can be provided to `train` method.
                            let sessionParameters = MLTrainingSessionParameters(
                                sessionDirectory: sessionDirectory,
                                reportInterval: 10,
                                checkpointInterval: 100,
                                iterations: 10
                            )

To the final method:

let trainJob = try MLActionClassifier.train(trainingData: trainingDataSource, parameters: modelParameters, sessionParameters: sessionParameters)

The job crashes saying it cannot find plist files. I notice that only one plist file is written: meta.plist

It seems there should also be a parameters.plist written, but it is not there.