null reference exception line 33

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

public class menuScript : MonoBehaviour
{
    public Canvas optionsMenu;
    public Canvas quitMenu;
    public Button startText;
    public Button optionsText;
    public Button quitText;
    public Slider slider;
    private float cachedVolume;
    private bool audioMuted = false;

    //the initial values for application launch, note that the quit and option menus are off by default
    void Start()
    {
        quitMenu = quitMenu.GetComponent<Canvas> ();
        optionsMenu = optionsMenu.GetComponent<Canvas> ();
        startText = startText.GetComponent<Button> ();
        optionsText = optionsText.GetComponent<Button>();
        quitText = quitText.GetComponent<Button> ();
        quitMenu.enabled = false;
        optionsMenu.enabled = false;
    }

    //update is called once per frame
     void Update()
    {
        if (!audioMuted)
        {
            AudioListener.volume = slider.value;
        }
    }

    //toggles global audio mute on and off.
    public void MuteAudio()
    {
        if (!audioMuted)
        {
            audioMuted = true;
            slider.enabled = false;
            cachedVolume = slider.value;
            slider.value = 0;
            AudioListener.volume = 0;
        }
        else
        {
            audioMuted = false;
            slider.enabled = true;
            slider.value = cachedVolume;
            AudioListener.volume = cachedVolume;
        }
    }

    // pauses audio
    public void PauseAudio()
    {
        AudioListener.pause = !AudioListener.pause;
    }

    // open up the options menu when clicking on options
    public void optionsPress()
    {
        optionsMenu.enabled = true;
        quitMenu.enabled = false;
        startText.enabled = false;
        optionsText.enabled = false;
        quitText.enabled = false;
    }

    //clicking on quit in the main menu, opens the quit confirmation window
    public void ExitPress()
    {
        quitMenu.enabled = true;
        optionsMenu.enabled = false;
        startText.enabled = false;
        optionsText.enabled = false;
        quitText.enabled = false;
    }

    //clicking cancel or no on options or exit menu. brings you back to main menu and reenables the main menu text
    public void NoPress()
    {
        quitMenu.enabled = false;
        optionsMenu.enabled = false;
        startText.enabled = true;
        optionsText.enabled = true;
        quitText.enabled = true;
    }

    /* clicking on play starts level 1 test map
     *  should eventually send player to character select
     *  and map select screen
     */
    public void StartLevel()
    {
        Application.LoadLevel(1);
    }

    //when confirming game quit, closes the application, on unity game scene window, this doesn't do anything
    // this is normal.
    public void ExitGame()
    {
        Application.Quit();
    }
}