Fix NullReferenceException in semantic highlighting.

ICSharpCode.NRefactory.CSharp/Analysis/SemanticHighlightingVisitor.cs

@@ -289,8 +289,11 @@ namespace ICSharpCode.NRefactory.CSharp.Analysis
 		{
 			if (member.EntityType != EntityType.Method || member.ReturnType.Kind != TypeKind.Void)
 				return false;
-			while (member.IsOverride)
+			while (member.IsOverride) {
 				member = (IParameterizedMember)InheritanceHelper.GetBaseMember(member);
+				if (member == null)
+					return false;
+			}
 			return IsInactiveConditional(member.Attributes);
 		}
 		

ICSharpCode.NRefactory.CSharp/Ast/AstNodeCollection.cs

@@ -28,7 +28,11 @@ namespace ICSharpCode.NRefactory.CSharp
 	/// <summary>
 	/// Represents the children of an AstNode that have a specific role.
 	/// </summary>
-	public class AstNodeCollection<T> : ICollection<T> where T : AstNode
+	public class AstNodeCollection<T> : ICollection<T>
+		#if NET_4_5
+		, IReadOnlyCollection<T>
+		#endif
+		where T : AstNode
 	{
 		readonly AstNode node;
 		readonly Role<T> role;