How to turn off/on a collider using a bool variable from another script

Hi, I'm new to C# and am struggling to get my head around some of the syntax.

What I want to do is turn off a capsule collider which is on my ground object, but only if the player is in a certain 'form'

I have created 2 Scripts.

One is for my Player. Within this player script there is a public bool called 'IsBlue'. This is toggled on/off with the return key.

I then have another script attached to a ground platform. This platform has 2 colliders. one for the actual surface of the platform which acts as the actual ground. Another which is a capsule collider which surrounds the platform. This Capsule collider has a material with Bounce properties attached to it. The Capsule collider is the one I want to switch off if the 'IsBlue' bool from my player script is true.

I have attached the relevant code from each script below.

My Players Script (The game object it's assigned to is called PlayerObj. The scripts name is Player):

   using UnityEngine;
   using System.Collections;

    public class Player : MonoBehaviour
     {

     //Bool to identify if blue
      public bool IsBlue = false;
   
 

    // Use this for initialization
    void Start()
    {
    }
 
     // Update is called once per frame
    void Update()
    {

        //button press to toggle blue on/off
        if (Input.GetKeyDown(KeyCode.Return))
        {
            IsBlue = !IsBlue;
        }
     }
   }

My Platform script(Attached to the game object Redplatform);

 using UnityEngine;
 using System.Collections;

 public class RedMagnet : MonoBehaviour
 {
      private CapsuleCollider capsuleCollider;
      private Player player;

 void Sart()
 {
     capsuleCollider = GetComponent<CapsuleCollider>();
     player = GameObject.FindGameObjectWithTag("PlayerObj").GetComponent<Player>();
 }

 void Update()
 {
     // check if the player is blue and do something
     if (player.IsBlue)
     {
         capsuleCollider.enabled = false;
     }
     else
     {
         capsuleCollider.enabled = true;
      }
    }
   }

The scripts have been attached to their relevant game objects and the game runs. However the capsule collider is not witching on or off? I made the 'IsBlue' bool public so I can see it toggling on/off. that is working. But the capsule collider is just staying on. The only error i'm getting is

'NullReferenceException: Object reference not set to an instance of an object RedMagnet.Update () (at Assets/Code/Scripts/RedMagnet.cs:18)'

RedMagnet.cs is the name of the script for the platform.

Any ideas?