NullReferenceException: Object reference not set to an instance of an object не получается исправить

Выбивает ошибку `NullReferenceException: Object reference not set to an instance of an object` в 2 местах

На этих строках находиться код rb.velocity = new Vector2(speed, rb.velocity.y); anim.SetBool("isJumping", false);

Не как не могу исправить ошибку

public float speed;

public float jumpForce;

public float normalSpeed;

private Rigidbody2D rb;
public Transform feetPos;

private bool facingRight = true;

private bool isGrounded;

public float checkRadius;

public LayerMask whatIsGround;

private Animator anim;

public void FixedUpdate()
{
    rb.velocity = new Vector2(speed, rb.velocity.y);
    {
        anim.SetBool("isRunning", true);
    }
}

public void OnButtonUp()
{
    speed = 0f;
    anim.SetBool("isRunning", false);
}

private void start()

{
    speed = 0f;
    anim = GetComponent<Animator>();
    rb = GetComponent<Rigidbody2D>();
}

public void OnLeftButtinDown()
{
    if (speed <= 0f)
    {
        speed = -normalSpeed;
        transform.eulerAngles = new Vector3(0, 180, 0);
    }
}
public void OnRightButtinDown()
{
    if (speed >= 0f)
    {
        speed = normalSpeed;
        transform.eulerAngles = new Vector3(0, 0, 0);
    }
}


private void Update()

{

    isGrounded = Physics2D.OverlapCircle(feetPos.position, checkRadius, whatIsGround);

    if (isGrounded == true)

    {
        anim.SetBool("isJumping", false);
    }
    else
    {

        anim.SetBool("isJumping", true);

    }
}

public void OnJumpButtonDown()

{

    if (isGrounded == true)

    {

        rb.velocity = Vector2.up * jumpForce;

        anim.SetTrigger("takeOf");

    }

}

}