Fix nullability of ITreeNode

ICSharpCode.ILSpyX/Abstractions/ITreeNode.cs

@@ -24,8 +24,8 @@ namespace ICSharpCode.ILSpyX.Abstractions
 {
 	public interface ITreeNode
 	{
-		object Text { get; }
-		object Icon { get; }
+		object? Text { get; }
+		object? Icon { get; }
 		IEnumerable<ITreeNode> Children { get; }
 
 		void EnsureLazyChildren();

ILSpy/TreeNodes/ILSpyTreeNode.cs

@@ -138,7 +138,7 @@ namespace ICSharpCode.ILSpy.TreeNodes
 			}
 		}
 
-		protected virtual void Settings_Changed(object sender, PropertyChangedEventArgs e)
+		protected virtual void Settings_Changed(object? sender, PropertyChangedEventArgs e)
 		{
 			if (sender is not ILSpy.LanguageSettings)
 				return;