Object reference not set to the instance of an object

Hey all, So, today I've been trying to program a timer for a game I'm making. In the game, the timer counts soun from 60 seconds to 0, and at 0, the game ends. Just as a placeholder for the moment ( because I can't seem to get it to count down in seconds) I have set the timer value to 60, and applied the scrit for the timer to a text so the value is seen visibly counting down. Here is the script I have:

  using UnityEngine;
  using UnityEngine.UI;
  using System.Collections;
  
  public class Timer : MonoBehaviour {
      public int timer;
      public Text timerText;
      public GameObject Timer
  
      // Use this for initialization
      void Start () {
          timer = 600;
  
      }
      
      // Update is called once per frame
      void Update () {
  
          timerText.text = timer.ToString (Timer);
          
          timer -= 1;
      }
  }

The error seems to happen line 18. It is indeed, rather annoying. Any and all help and pointers would be greatly appreciated, thanks :)