Error NullReferenceException: Object reference not set to an instance of an object mouseball.setcounttext ()

Question

You don't have to read all the code, my question is a bit further down. Thank you for helping :s

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

public class mouseball : MonoBehaviour
{
    public float speed;
    private int count;
    public Text countText;
    // Use this for initialization
    void Start()
    {
        count = 0;
        setcounttext();

    }

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

        Vector3 movement = new Vector3(moveHorizontal, 0, moveVertical);

        GetComponent<Rigidbody>().AddForce(movement * speed * Time.deltaTime);
    }

    void OnTriggerEnter(Collider other)
    {
        Debug.Log("trigger");
        if (other.gameObject.tag == "pickup")
        {
            other.gameObject.SetActive(false);
            count = count + 1;
            setcounttext();
        }
    }

    void setcounttext()
    {
        countText.text = "Count : " + count.ToString();
    }

}

The lines that the error is referencing to are:

void setcounttext()
{
    countText.text = "Count : " + count.ToString();
}

I tried but couldn't find the solution which is prob are easy one, I'm grateful for any help.