我设置战斗文本的脚本给出了例外

我的TextManager脚本是这样的

using UnityEngine;
using System.Collections;
public class TextManager : MonoBehaviour {
private static TextManager instance;
public RectTransform canvasTransform;
public GameObject textPrefab;
public float speed;
public Vector3 direction;

void Start() {
    speed = 1f;
    direction = (new Vector3(0,1,0));
}
public static TextManager Instance
{
    get
    {
        if (instance == null)
        {
            instance = GameObject.FindObjectOfType<TextManager>();
        }
        return instance;
    }
       }

   public void CreateText(Vector3 position)
   {
    GameObject sct  =  (GameObject)Instantiate(textPrefab, position, Quaternion.identity);
    sct.transform.SetParent(canvasTransform);
    sct.GetComponent<RectTransform>().localScale = new Vector3(0.08f, 0.08f, 0.08f);
    sct.GetComponent<CombatText>().Initialize(speed, direction);
}
}

战斗文本是这样的

using System.Collections;
public class CombatText : MonoBehaviour {
private float speed;
private Vector3 direction;
private float fadeTime;
// Use this for initialization
void Start () {
}
// Update is called once per frame
void Update () {
    float translation = speed * Time.deltaTime;
    transform.Translate(direction * translation);
}
public void Initialize(float speed, Vector3 direction) {
    this.speed = speed;
    this.direction = direction;

}
}

这就是我称之为TextManager.Instance.CreateText（transform.position）的方式;我不知道为什么它会给出 NullReferenceException：对象引用未设置为对象的实例TextManager.CreateText （Vector3 位置） （at Assets/TextManager/TextManager.cs：37）PlayerControl.Update （） （at Assets/Player/PlayerControl.cs：35）

有什么提示吗？