- add device backing field

Mystify/ViewModels/MainWindowViewModel.cs

@@ -13,6 +13,8 @@ namespace Mystify.ViewModels
 {
     public class MainWindowViewModel : ViewModelBase
     {
+        private readonly IDevice _device;
+        
         public MainWindowViewModel()
         {
             if (!IsInDesignMode)
@@ -24,10 +26,10 @@ namespace Mystify.ViewModels
         
         public MainWindowViewModel(IDeviceFactory deviceFactory, IControllableCollectorFactory controllableCollectorFactory)
         {
-            IDevice device = deviceFactory.Create();
+            _device = deviceFactory.Create();
             IControllableCollector controllableCollector = controllableCollectorFactory.Create();
 
-            Channels = device.Channels;
+            Channels = _device.Channels;
             Controllables = controllableCollector.Controllables;
         }