Fixed error caused by Root role.

Ok now I know why it needs to be there :)

ICSharpCode.NRefactory.CSharp/Ast/AstNode.cs

@@ -35,6 +35,11 @@ namespace ICSharpCode.NRefactory.CSharp
 {
 	public abstract class AstNode : AbstractAnnotatable, PatternMatching.INode
 	{
+		/// <summary>
+		/// Root of an abstract syntax tree.
+		/// </summary>
+		public static readonly Role<AstNode> Root = new Role<AstNode> ("Root");
+		
 		#region Null
 		public static readonly AstNode Null = new NullAstNode ();
 		
@@ -124,7 +129,7 @@ namespace ICSharpCode.NRefactory.CSharp
 		AstNode nextSibling;
 		AstNode firstChild;
 		AstNode lastChild;
-		Role role = Roles.Root;
+		Role role = Root;
 		
 		public abstract NodeType NodeType {
 			get;

ICSharpCode.NRefactory.CSharp/Ast/Roles.cs

@@ -30,10 +30,6 @@ namespace ICSharpCode.NRefactory.CSharp
 {
 	public static class Roles
 	{
-		/// <summary>
-		/// Root of an abstract syntax tree.
-		/// </summary>
-		public static readonly Role<AstNode> Root = new Role<AstNode> ("Root");
 			
 		// some pre defined constants for common roles
 		public static readonly Role<Identifier> Identifier = new Role<Identifier> ("Identifier", CSharp.Identifier.Null);

ICSharpCode.NRefactory/Role.cs

@@ -70,6 +70,8 @@ namespace ICSharpCode.NRefactory
 		{
 			if (name == null)
 				throw new ArgumentNullException("name");
+			if (nullObject == null)
+				throw new ArgumentNullException ("nullObject");
 			this.nullObject = nullObject;
 			this.name = name;
 		}