How to fill a circle image by every second

Hi! I came back with a project for fun but i need help. Every time i press build is showing to me this error:

• NullReferenceException: Object reference not set to an instance of an object AceDeCeas.Update () (at Assets/Scripts/AceDeCeas.cs:23)

Here you have my source

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;

 public class AceDeCeas : MonoBehaviour {

 float time=0;
 public Image secundar;
 public Image minutar;
 public Image orar;
 public Slider slider1;
 void Start () {
     secundar = GetComponent<Image> ();
     minutar= GetComponent<Image> ();
     orar = GetComponent<Image> ();
     slider1 = GetComponent<Slider> ();
     secundar.fillAmount = 0;
 }
 
 // Update is called once per frame
 void Update ()
 {time += Time.deltaTime;
     secundar.fillAmount = Mathf.Clamp (time, 0, 1);
 }

}

First I wanted to make something to work with a slider but still error was showing.

I'll really appreciate the help :)

Serju