Error when extending GridGraph

Hello, I am new to the forum and A* Pathfinding Project. I am attempting to extend GridGraph to enable custom collision implementation. I am using Rotorz Tile System. I would like to say that A* Pathfinding Project is a great feat of craftsmanship, thank you.

I am experiencing an error:
NullReferenceException: Object reference not set to an instance of an object
Pathfinding.GridGraphEditor.OnSceneGUI (Pathfinding.NavGraph target) (at Assets/AstarPathfindingProject/Editor/GraphEditors/GridGeneratorEditor.cs:629)

I have searched for any explanation or hint as to the origin of my mistake. I am sure it is simply something I overlooked. I appreciate any assistance that can be offered.

I have created the below two classes.

Class 1…

using UnityEngine;
using UnityEditor;
using Pathfinding;
using System.Collections;

[CustomGraphEditor (typeof(RotorzGraph),“Rotorz Graph”)]
public class RotorzGraphEditor : GridGraphEditor {

    public override void OnInspectorGUI (NavGraph target) 
	RotorzGraph graph = target as RotorzGraph;



and Class 2…

using UnityEngine;
using System.Collections.Generic;
using Pathfinding;

public class RotorzGraph : Pathfinding.GridGraph {


FYI for those using Rotorz and are beginners. Simply use box colliders on your solid brushes and the A* project will pick them up as obstacles. The brushes need to be set to the same layer that the Ray collision detection is scanning within your A* game object. I have not tested the performance cost of having many box colliders. It may be possible to increase the size of the brush, so that for a rectangular set of tiles will represent 1 box collider.