NullRefenceException error

So I get “NullReferenceException: Object reference not set to an instance of an object” at this line camera3rd.enabled=false;. Can’t figure out what’s wrong

  using UnityEngine;
    using System.Collections;
    public class CameraSwitch : MonoBehaviour {
    	public SmoothFollow camera3rd;
    	public Transform target;
    	public float time;
    	void Start () {
    		// Auto setup smoothfollow to a cameraswitch script
    		camera3rd = gameObject.GetComponent<SmoothFollow>() ;
//disabling this script

public SmoothFollow camera3rd;

This is an uninitialized variable, did you forget to add a SmoothFollow script component to the GameObject?