Nullreference?

@MenuItem("Level Lord/Gridify")
    static function CreatePrefab() {
        var objs = Selection.gameObjects;
        for (var go : GameObject in objs) {
            var localPath : String = "Assets/Level.prefab";
            generate(go, localPath);
        }
    }

static var flippy : boolean = false;

    static function generate(obj : GameObject, localPath : String) {
        var gridify : GameObject;
        var prefab : Object = PrefabUtility.CreateEmptyPrefab(localPath);
        for (i = 0; i < 10; i++) {
            flippy = !flippy;
            for (j = 0; j < 10; j++) {
                var clone : GameObject = PrefabUtility.InstantiatePrefab (obj) as GameObject;
                if (flippy)
                    clone.transform.position = Vector3 (i*1.5, 0, j*1.7272);
                else
                    clone.transform.position = Vector3 (i*1.5, 0, j*1.7272 + 0.8636);
                clone.transform.parent = gridify.transform;
}}      PrefabUtility.ReplacePrefab(gridify, prefab, ReplacePrefabOptions.ConnectToPrefab);
}