The referenced script on this Behavior (GameObject 'Sphere') is missing!

Appeared when i tried running a script to roll a sphere around on a plane. My sphere name is ‘Sphere’ and the c# file is named PlayerController. Whats wrong here?

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);
}

}

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour {

 public float speed;
 private Rigidbody rb;
 void Start()
 {
     rb = gameObject.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);
 }
}