- adjust to nullable

Lib.Driver/Xml/XmlButton.cs

@@ -10,6 +10,6 @@ namespace Lib.Driver.Xml
     public class XmlButton
     {
         [XmlElement("Type")]
-        public string ButtonType { get; set; }
+        public string? ButtonType { get; set; }
     }
 }

Lib.Driver/Xml/XmlChannel.cs

@@ -11,13 +11,13 @@ namespace Lib.Driver.Xml
     public class XmlChannel
     {
         [XmlElement("Fader")]
-        public XmlFader Fader { get; set; }
+        public XmlFader? Fader { get; set; }
         
         [XmlElement("Knob")]
-        public XmlKnob Knob { get; set; }
+        public XmlKnob? Knob { get; set; }
 
         [XmlArray("Buttons")]
         [XmlArrayItem("Button", typeof(XmlButton))]
-        public List<XmlButton> Buttons { get; set; }
+        public List<XmlButton>? Buttons { get; set; }
     }
 }