NullReferenceException: Object reference not set to an instance of an object PlayerController.FixedUpdate () (at Assets/Scripts/PlayerController.cs:22)

So i know that its telling me that there’s a problem in the script PlayerController in the section
fixedupdate on line 22 but when i go to the source i dont see what is wrong with the code it looks ok to me
here is the code oh yeh im a newbie but im getting there:) also thank you for your time and help:)

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour
{
public float speed;

private Rigidbody rb;

void start()
{
    rb = GetComponent<Rigidbody>();
}

void FixedUpdate()
{
    float moveHorizontal = Input.GetAxis("Horizontal");
    float moveVertical = Input.GetAxis("Vertical");

    Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);

    rb.AddForce (movement * speed);
}

}

You have to write Start with a capital S. So Start() instead of start(). Or Unity will not call the method by default.