camera switching problem

Unity Engine
1

Hello guys !
Am pretty new to unity programming and got stuck lately by an annoying bug. Searched a lot on the internet but maybe I am too stupid to understand what I am doing wrong or maybe it’s just not in there. (Pity, help me)

So here is the things. I am using a UFPS camera to travel in my scene and a second camera, not moving at all, to display a clickable UI.
So i put a script on a wall for instance, when the guy click on the wall, it switches camera and show him some buttons to change the wall textures.

My bug is that, when I click on the wall it says me this :
NullReferenceException: Object reference not set to an instance of an object
Canvas_Activer.Start () (at Assets/Scripts/Canvas_Activer.cs:24)
(canvas activer being my script)

using UnityEngine;
using System.Collections;

public class Canvas_Activer : MonoBehaviour {

    public string CanvasName;
    public int Timer;
    public float Speed=1.0f;

    private GameObject Canvas;
    private Canvas CanvasComponent;
    private GameObject Maincamera;
    private Camera MovingCamera;
    private float timing=0.0f;
    private bool flag=false;
    private float startTimer;
   

    // Use this for initialization
    void Start () {
        GameObject Canvas = GameObject.Find(CanvasName);
        GameObject Maincamera = GameObject.Find("FPSCamera");
        CanvasComponent = Canvas.GetComponent<Canvas>();
        MovingCamera = Maincamera.GetComponent<Camera>() as Camera;
    }
   
    void Update () {
        if(flag == true){
            timing = Time.time - startTimer;
            Screen.showCursor = true;
            Debug.Log(timing);
            Maincamera.camera.enabled=false;

            if(timing>Timer){
                CanvasComponent.enabled=false;
                Screen.showCursor = false;
                flag = false;
            }
        }
    }
   

    void OnMouseOver(){
        Screen.showCursor = true;
        if(Input.GetButton("Fire1")){
            CanvasComponent.enabled=true;
            startTimer = Time.time;
            flag = true;
        }
    }

    }

Tryed out a lot of things but… just can’t do it :frowning:
If someone could help me, I would really be gratefull

Thanks all !

2

AM really struggling with this.