Timer called in wrong interface

I have a Problem with my timer in C#. The Problem is following:

The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))

Can you help me and solve this? I have no plan.

Thank you. Here is my Code:

using System;
using Windows.ApplicationModel.Background;
using Windows.Foundation;
using Windows.UI.Xaml;
using Windows.Devices.Gpio;
using System.Diagnostics;
namespace StepperBackgroundV3
{
public sealed class StartupTask : IBackgroundTask
{
    private const int Stepper_PIN_A = 18;
    private const int Stepper_DIR_A = 27;
    private const int Sleep_PIN_A = 13;
    private const int Enable_PIN_A = 16;
    private const int Fault_PIN_A = 24;


    private GpioPin StepperDirA;
    private GpioPin StepperPinA;


    private GpioPin FaultPinA;
    private GpioPin SleepPinA;
    private GpioPin EnablePinA;

    private DispatcherTimer timer;
    private double BEAT_PACE = 1000;
    private double CounterClockwiseDanceMove = 1;
    private double ClockwiseDanceMove = 2;
    private double currentDirection;
    private double PulseFrequency = 20;
    Stopwatch stopwatch;

    public void Run(IBackgroundTaskInstance taskInstance)
    {
        //InitializeComponent();

        //
        //  !!! Wieder einbauen zum Starten der Motoren !!!
        //
        this.InitDancing();
        //
        //
        //pausiereMotoren();
        //erwarteAnweisungen();
    }
    private void InitDancing()
    {
        try
        {
            var gpio = GpioController.GetDefault();

            if (gpio == null)
            {
                StepperPinA = null;
                return;
            }

            StepperPinA = gpio.OpenPin(Stepper_PIN_A);
            StepperPinA.SetDriveMode(GpioPinDriveMode.Output);

            StepperDirA = gpio.OpenPin(Stepper_DIR_A);
            StepperDirA.SetDriveMode(GpioPinDriveMode.Output);


            FaultPinA = gpio.OpenPin(Fault_PIN_A);
            FaultPinA.SetDriveMode(GpioPinDriveMode.Input);

            SleepPinA = gpio.OpenPin(Sleep_PIN_A);
            SleepPinA.SetDriveMode(GpioPinDriveMode.Output);


            EnablePinA = gpio.OpenPin(Enable_PIN_A);
            EnablePinA.SetDriveMode(GpioPinDriveMode.Output);


            stopwatch = Stopwatch.StartNew();

            currentDirection = 0; // Initially we aren't dancing at all.

            timer = new DispatcherTimer();
            timer.Interval = TimeSpan.FromMilliseconds(BEAT_PACE);
            timer.Tick += Beat;

            if (StepperPinA != null)
            {
                timer.Start();
                Windows.System.Threading.ThreadPool.RunAsync(this.MotorThread, Windows.System.Threading.WorkItemPriority.High);
            }
        }catch(Exception e)
        {

        }

    }


    private void Beat(object sender, object e)
    {
        if (currentDirection != ClockwiseDanceMove)
        {
            currentDirection = ClockwiseDanceMove;
            //GpioStatus.Text = "Yay!";
        }
        else
        {
            currentDirection = CounterClockwiseDanceMove;
            //GpioStatus.Text = "Windows 10!";
        }
    }

    private void MotorThread(IAsyncAction action)
    {
        while (true)
        {
            if (currentDirection != 0)
            {
                StepperPinA.Write(GpioPinValue.High);
            }
            // Könnten es noch heruntersetzen auf 0.1 ms
            Wait(currentDirection);

            StepperPinA.Write(GpioPinValue.Low);
            Wait(PulseFrequency - currentDirection);
        }
    }

    private void Wait(double milliseconds)
    {
        long initialTick = stopwatch.ElapsedTicks;
        long initialElapsed = stopwatch.ElapsedMilliseconds;
        double desiredTicks = milliseconds / 1000.0 * Stopwatch.Frequency;
        double finalTick = initialTick + desiredTicks;
        while (stopwatch.ElapsedTicks < finalTick)
        {

        }
    }

    //Standbymode
    private void pausiereMotoren()
    {
        //Sleep setzen....
    }

    //Communicationssocket
    private void erwarteAnweisungen()
    {
        //Clientsocket Coden


        //Irgendwie den Start der Motoren aufrufen
        //this.InitDancing();
    }

    //Fehlererkennung
    private void Fault()
    {
        if(FaultPinA.Read() == GpioPinValue.Low)
        {
            //LED und Summer ansteuern
            //String an Communicationlogger MotorA-Fehler
            pausiereMotoren();
        }

        //if (FaultPinB.Read() == GpioPinValue.Low)
        //{
        //    //LED und Summer ansteuern
        //    //String an Communicationlogger MotorB-Fehler
        //    pausiereMotoren();
        //}
    }

}
}