在Spawn期间，对象引用未设置为Object的实例

Question

我不知道出了什么问题。调试日志说

NullReferenceException: Object reference not set to an instance of an object
Vuforia.ZoneManagerScript.Spawn () (at Assets/Yugioh/ZoneManagerScript.cs:31)
Vuforia.Spawn.OnTrackableStateChanged (Status previousStatus, Status newStatus) (at Assets/Yugioh/Spawn.cs:47)
Vuforia.TrackableBehaviour.OnTrackerUpdate (Status newStatus)
Vuforia.DataSetTrackableBehaviour.OnTrackerUpdate (Status newStatus)
Vuforia.StateManagerImpl.UpdateTrackablePoses (UnityEngine.Transform arCameraTransform, .TrackableResultData[] trackableResultDataArray, Int32 originTrackableID, Int32 frameIndex)
Vuforia.VuforiaManagerImpl.UpdateTrackers (FrameState frameState)
Vuforia.VuforiaManagerImpl.Update (ScreenOrientation counterRotation, Boolean reapplyOldState)
Vuforia.VuforiaAbstractBehaviour.UpdateStatePrivate (Boolean forceUpdate, Boolean reapplyOldState)
Vuforia.VuforiaAbstractBehaviour.Update ()".

相关代码片段：

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

namespace Vuforia
{
    public class ZoneManagerScript : MonoBehaviour

    {

        public Vector3 Zone1L;
        public Vector3 Zone2L;
        public Vector3 Zone3L;
        public Vector3 Zone4L;
        public Vector3 Zone5L;
        GameObject SpawnedM;
        Spawn SpawnScript;
        GameObject monster;


            public void Spawn () 
            {
                GameObject MonsterONField = GameObject.FindGameObjectWithTag ("Monster");
                SpawnScript = MonsterONField.GetComponent<Spawn>();
                monster = SpawnScript.Monster;

                Dictionary <Vector3,bool> IsZoneEmpty = new Dictionary <Vector3,bool>();

                IsZoneEmpty.Add(Zone1L,true);

                if (IsZoneEmpty [Zone1L] == true)
                {
                    SpawnedM = Instantiate(monster,Zone1L,Quaternion.identity) as GameObject;
                    Debug.Log("MonsterSpawned");
                    IsZoneEmpty[Zone1L]=false;
                }

            }

    }
}

片段2：

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

namespace Vuforia
{
    public class Spawn : MonoBehaviour,
    ITrackableEventHandler

    {


        private TrackableBehaviour mTrackableBehaviour;
        public GameObject Monster;                // The enemy prefab to be spawned.
        GameObject Zonemanager;
        ZoneManagerScript ZMS; 

         void Start()
        {
            mTrackableBehaviour = GetComponent<TrackableBehaviour> ();
            if (mTrackableBehaviour) 
            {
                mTrackableBehaviour.RegisterTrackableEventHandler (this);

            }

        }

            public void OnTrackableStateChanged(
                TrackableBehaviour.Status previousStatus,
                TrackableBehaviour.Status newStatus)

            {

                if (newStatus == TrackableBehaviour.Status.DETECTED ||
                    newStatus == TrackableBehaviour.Status.TRACKED ||
                    newStatus == TrackableBehaviour.Status.EXTENDED_TRACKED) 
                    {
                        Zonemanager = GameObject.FindGameObjectWithTag ("Zone Manager");
                        ZMS = Zonemanager.GetComponent<ZoneManagerScript> ();
                        ZMS.Spawn();

                    } 
                        else 
                            {

                            }
            }
    }
}