Windows 10 IoT, UWP und der SerialPort (NullReferenceException)

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using System.Threading;
using System.Threading.Tasks;
using Windows.Devices.Enumeration;
using Windows.Devices.SerialCommunication;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.Storage.Streams;
using Windows.UI.Popups;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

namespace SerialMonitor_UWP__2
{
    /// <summary>
    /// Eine leere Seite, die eigenständig verwendet oder zu der innerhalb eines Rahmens navigiert werden kann.
    /// </summary>
    public sealed partial class MainPage : Page
    {
        private SerialDevice serialPort = null;
        DataWriter dataWriteObject = null;
        DataReader dataReaderObject = null;

        private ObservableCollection<DeviceInformation> listOfDevices;
        private CancellationTokenSource ReadCancellationTokenSource;

        public MainPage()
        {
            this.InitializeComponent();
            listOfDevices = new ObservableCollection<DeviceInformation>();
            ListAvailablePorts();
        }

        private async void ListAvailablePorts()
        {
            try
            {
                string aqs = SerialDevice.GetDeviceSelector();
                var dis = await DeviceInformation.FindAllAsync(aqs);
                
                for (int i = 0; i < dis.Count; i++)
                {
                    listOfDevices.Add(dis[i]);
                }

                DeviceListSource.Source = listOfDevices;
                listBox.SelectedIndex = -1;

            }
            catch (Exception ex)
            {
                textBox.Text = ex.Message;
                textBox.Text += "ListAvailablePorts()";
                listBox.SelectedIndex = -1;
            }

            serialPort = null;
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            ListAvailablePorts();
        }

        private async void button2_Click(object sender, RoutedEventArgs e)
        {
            var selection = listBox.SelectedItems;

            if(selection.Count <= 0)
            {
                textBox.Text = "Wählen Sie einen COM-Port und verbinden Sie";
                return;
            }

            DeviceInformation entry = (DeviceInformation)selection[0];

            try
            {
                serialPort = await SerialDevice.FromIdAsync(entry.Id);
                serialPort.WriteTimeout = TimeSpan.FromMilliseconds(1000);
                serialPort.ReadTimeout = TimeSpan.FromMilliseconds(1000);
                serialPort.BaudRate = 9600;
                serialPort.Parity = SerialParity.None;
                serialPort.StopBits = SerialStopBitCount.One;
                serialPort.DataBits = 8;
                serialPort.Handshake = SerialHandshake.None;

                textBox.Text = "COM-Port erfolgreich eingerichtet: ";
                textBox.Text += serialPort.PortName + "-";
                textBox.Text += serialPort.BaudRate + "-";
                textBox.Text += serialPort.DataBits + "-";
                textBox.Text += serialPort.Parity.ToString() + "-";
                textBox.Text += serialPort.StopBits;

            }
            catch(Exception ex)
            {
                textBox.Text = ex.Message;
            }

        }
    }
}