Countdown Timer

I am trying to make a script that counts down the timer and stays in the same place and keeps counting down through 3 levels. I wrote this:
using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class countdownTimer : MonoBehaviour {
public Text score;
public GameObject countdownManager;

void Update()

void setTime()
    float timeSinceStart = Mathf.Round(Time.time);
    float scoreText = (25 - timeSinceStart);
    if (scoreText > 0) {
        score.text = (scoreText.ToString());
        score.text = ("0");


When switching, I got errors saying:
The object of type ‘Text’ has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.

Can anyone fix the error?

Mark the score like this: DontDestroyOnLoad(score.gameObject);

Thank You! The program works with no errors.