fix(MMNotificationClient): Fix possible crash when closing the app

Fixes #1201

Author: Belphemur

SoundSwitch/Framework/NotificationManager/MMNotificationClient.cs

@@ -174,13 +174,14 @@ public void OnPropertyValueChanged(object sender, DevicePropertyChangedEventArgs
 
         public void Dispose()
         {
-            _notificationClient.DeviceAdded -= OnDeviceAdded;
-            _notificationClient.DeviceRemoved -= OnDeviceRemoved;
-            _notificationClient.DevicePropertyChanged -= OnPropertyValueChanged;
-            _notificationClient.DeviceStateChanged -= OnDeviceStateChanged;
-            _notificationClient.DefaultDeviceChanged -= OnDefaultDeviceChanged;
+            //If register wasn't called, _notificationClient can be null
+            _notificationClient?.DeviceAdded -= OnDeviceAdded;
+            _notificationClient?.DeviceRemoved -= OnDeviceRemoved;
+            _notificationClient?.DevicePropertyChanged -= OnPropertyValueChanged;
+            _notificationClient?.DeviceStateChanged -= OnDeviceStateChanged;
+            _notificationClient?.DefaultDeviceChanged -= OnDefaultDeviceChanged;
             _notificationClient = null;
             _mmDeviceEnumerator = null;
         }
     }
-}
+}