Rolling ball Text error. Removes itself when played

Here is my code for the text. Basically when I drag countText into the playerController component the program will originally accept this. However once the game is played it will remove the countText from the playerController (the box showing that there is no text selected). Also, once you exit play mode the CountText will return to the original position Can someone help me have text that counts the score for each ball collected

here is the script used:

using UnityEngine; using UnityEngine.UI; using System.Collections;

public class PlayerController : MonoBehaviour {

 public float speed;
 public Text countText;
 //public Text winText;
 
 private Rigidbody rb;
 private int count;
 
 void Start ()
 {
     rb = GetComponent<Rigidbody>();
     count = 0;
     SetCountText ();
     //winText.text = "";
 }
 
 void FixedUpdate ()
 {
     float moveHorizontal = Input.GetAxis ("Horizontal");
     float moveVertical = Input.GetAxis ("Vertical");
     
     Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
     
     rb.AddForce (movement * speed);
 }
 
 void OnTriggerEnter(Collider other) 
 {
     if (other.gameObject.CompareTag ( "Pick Up"))
     {
         other.gameObject.SetActive (false);
         count = count + 1;
         SetCountText ();
     }
 }
 
 void SetCountText ()
 {
     countText.text = "Count: " + count.ToString ();
     //if (count >= 12)
     {
         //winText.text = "You Win!";
     }
 }

}

[1]: /storage/temp/47207-screen-shot-2015-05-28-at-93036-am.png