Fix things after testing with virtual serial port

PamhagenSysCtrl/Helpers/PamhagenPlant.cs

@@ -1,9 +1,10 @@
-using System.Windows;
+using System.IO;
+using System.Windows;
 
 namespace PamhagenSysCtrl.Helpers {
     public class PamhagenPlant : IDisposable {
 
-        protected PamhagenPlc Plc;
+        protected PamhagenPlc? Plc;
 
         protected bool IsRunning = true;
         protected Thread? BackgroundThread;
@@ -61,6 +62,7 @@ namespace PamhagenSysCtrl.Helpers {
         }
 
         public async Task StartThread() {
+            if (Plc == null) throw new Exception();
             await Plc.TestConnection();
             Actuators = await Plc.ReadOutputs();
             BackgroundThread = new Thread(new ParameterizedThreadStart(BackgroundLoop));
@@ -71,12 +73,12 @@ namespace PamhagenSysCtrl.Helpers {
             IsRunning = false;
             BackgroundThread?.Interrupt();
             BackgroundThread?.Join();
-            Plc.Dispose();
+            Plc?.Dispose();
             GC.SuppressFinalize(this);
         }
 
         protected async void BackgroundLoop(object? parameters) {
-            while (IsRunning) {
+            while (IsRunning && Plc != null) {
                 try {
                     if (ActuatorsChanged) {
                         await Plc.WriteOutputs(Actuators);
@@ -90,10 +92,16 @@ namespace PamhagenSysCtrl.Helpers {
                     var str = "Bei der SPS ist ein Fehler aufgetreten:\n\n" + exc.Message;
                     if (exc.InnerException != null) str += "\n\n" + exc.InnerException.Message;
                     MessageBox.Show(str, "SPS-Fehler", MessageBoxButton.OK, MessageBoxImage.Error);
+                    if (exc is TimeoutException or IOException) {
+                        Plc?.Dispose();
+                        Plc = null;
+                        IsRunning = false;
+                    }
                 }
             }
             try {
-                await Plc.WriteOutputs(new());
+                if (Plc != null)
+                    await Plc.WriteOutputs(new());
             } catch (Exception exc) {
                 var str = "Beim Schließen der Verbindung zur SPS ist ein Fehler aufgetreten:\n\n" + exc.Message;
                 if (exc.InnerException != null) str += "\n\n" + exc.InnerException.Message;