NullReferenceException: Object reference not set to an instance of an object ( UI.text ) ?

Hi :
i want to change a ui text from scripting but i get this error i mentioned in the question and this is my script :

    using UnityEngine;
    using System.Collections;
    using UnityEngine.UI;
    public class MyScript : MonoBehaviour 
    public Text livscoretext;
    	void Start () 
    		livscoretext = GetComponent<Text>();
    		livscoretext.text = "score" ;

i solved it one minute after posting my question !

// i just removed  
livscoretext = GetComponent<Text>();
//and used only
livscoretext .text = "score";