NullReferenceException: Object reference not set to an instance of an object

NullReferenceException: Object reference not set to an instance of an object menuScript.Start () (at Assets/Scripts/menuScript.cs:15)

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class menuScript : MonoBehaviour {
 
     public Canvas quitMenu;
     public Button startText;
     public Button exitText;
     // Use this for initialization
     void Start ()
     
     {
         quitMenu = quitMenu.GetComponent<Canvas> ();
         startText = startText.GetComponent<Button> ();
         exitText = exitText.GetComponent<Button> ();
         quitMenu.enabled = false;
     }
     
     public void ExitPress()
 
     {
         quitMenu.enabled = true;
         startText.enabled = false;
         exitText.enabled = false;
     }
 
     public void NoPress()
 
     {
         quitMenu.enabled = false;
         startText.enabled = true;
         exitText.enabled = true;
     }
 
     public void StartLevel()
 
     {
         Application.LoadLevel (1);
     }
 
     public void ExitGame()
     
     {
         Application.Quit();
     }
 }