Hi, I'm making a doodle jump style game as my first unity project. The game begins and the platform boosts your jump speed and gets destroyed when you hit it which is fine but unity is returning the following error and no more platforms are spawned. From what I've read i need to set the destroyed platform to null, but im not sure how to do this...Please help!

"MissingReferenceException: The object of type 'Transform' has been destroyed but you are still trying to access it."

using UnityEngine;
using System.Collections;

public class Playermovement : MonoBehaviour
{
    public float movementSpeed = 8.0f;


    void Update() {
        rigidbody.velocity = new Vector3(0, rigidbody.velocity.y, 0); //Set X and Z velocity to 0
 
        transform.Translate(Input.GetAxis("Horizontal") * Time.deltaTime * movementSpeed, 0, 0);
	}

    void Jump() //Increases velocity of player when called.
    {
        rigidbody.velocity = new Vector3(0, 0, 0);
        rigidbody.AddForce(new Vector3(0, 700, 0), ForceMode.Force);        
    }
    void OnCollisionEnter(Collision collision){
    if(collision.gameObject.tag == "platform")
    {
	    Jump ();
        Destroy(collision.gameObject); 
    }
}