Fix multiple poller situational problems

Examples.WPF/ViewModels/AppViewModel.cs

@@ -0,0 +1,30 @@
+using MewtocolNet;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Examples.WPF.ViewModels {
+
+    public class AppViewModel : ViewModelBase {
+
+        private IPlc? plc;
+
+        public bool PlcIsNull => plc == null;
+
+        public bool PlcIsNotNull => plc != null;
+
+        public IPlc? Plc {
+            get => plc; 
+            set {
+                plc = value;
+                OnPropChange();
+                OnPropChange(nameof(PlcIsNull));
+                OnPropChange(nameof(PlcIsNotNull));
+            }
+        }
+
+    }
+
+}