HealthKit - “Stopped workout session cannot be restarted”

I am using the following code to record the heart rate from the internal watch sensor, which works fine! But when I stop the workout and want to start a new I am getting the error: workoutSession didFailWithError = Error Domain=com.apple.healthkit Code=3 "Stopped workout session cannot be restarted" UserInfo={NSLocalizedDescription=Stopped workout session cannot be restarted}

I think I have to clear in any kind the HKWorkoutSession *workoutSession and init it new but I don't know how?

Here is the code I am using:

@interface InterfaceController()

{

ExtensionDelegate *extensionDelegate;

/

HKHealthStore *healthStore;

HKWorkoutSession *workoutSession;

HKUnit *heartRateUnit;

HKQueryAnchor *anchor;

}

healthStore = [[HKHealthStore alloc] init];

workoutSession =[[ HKWorkoutSession alloc] initWithActivityType:HKWorkoutActivityTypeCrossTraining locationType:HKWorkoutSessionLocationTypeIndoor];

heartRateUnit = [HKUnit unitFromString:@"count/min"];

anchor = [HKQueryAnchor anchorFromValue:HKAnchoredObjectQueryNoAnchor];

workoutSession.delegate = self;

[healthStore startWorkoutSession:workoutSession];

How can I clear the workout session after stopping it for starting a new workout session:

[healthStore endWorkoutSession:workoutSession];

Not working:

workoutSession = nil;