Handling Undo/REdo in EditorWindow when editing DB record.

Unity: 4.6.3f1 on OsX. I am building a dialog editor window into Unity, when the editor opens up the user can choose a dialog to edit. Once a dialog is selected my code loads the dialog from a SQLite3 database and news a dialog model object.

The EditorWindow then displays the GUI elements for the fields in the dialog model. My problem seems to be that generic Objects can't be used in the Unity's undo/redo system.

 [Serializable()]
 public class DialogModelNode : UnityEngine.Object
 {
 ...
 [SerializeField]
     private string                  _title;                 // the title of this node
 ...
  public  string  title           { get {return _title;}          set { dirty |= (string.Compare(value, _title) != 0);            _title = value;}}
 ...
 }

Editor Code

 String val;
 EditorGUILayout.BeginHorizontal();
             EditorGUILayout.LabelField(new GUIContent("(" + root.rootID + ")Title:","The title of this Dialog (Designer only)"), GUILayout.Width( 60));
             EditorGUI.BeginChangeCheck();
             val = EditorGUILayout.TextField(root.root.title);
             if (EditorGUI.EndChangeCheck ())
             {
                 Debug.Log("Undo " + root.root != null);
                 Undo.RecordObject (root.root, "Edit Dialog Title");
                 root.root.title = val;
                 EditorUtility.SetDirty (root.root);
             }
 
         EditorGUILayout.EndHorizontal();

When this code executes I get the following error "Undo object may not be null." even though the preceding log statement shows true.

What is the best way to handle Undo/Redo in this case? Is it possible to create by own Undo/Redo records and get events called on it when they are undone or redone?

Any help would be useful, I have been googling and have found several examples un Undo/Redo code but they don't seem to function in this case and I am not sure why?

Thanks - Robert.