NullReferenceException with StartCoroutine()

Question

I made coroutine which will do my loading in game but every time i click play button I get NullReferenceException on line: StartCoroutine(LoadNewScene());

Here is my full code:

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

public class Loading : MonoBehaviour
{

    private bool LoadScene = false;

    [SerializeField]
    private int scene = 0;
    [SerializeField]
    private Text LoadingText;
    [SerializeField]
    private Text TipText;

    // Use this for initialization
    void Start()
    {

    }

    // Update is called once per frame
    void Update()
    {
        if (LoadScene == true)
        {
            scene++;
        }
    }

    public void Load()
    {
        LoadScene = true;
        //LoadingText.color = new Color(LoadingText.color.r, LoadingText.color.g, LoadingText.color.b, Mathf.PingPong(Time.time, 1));
        StartCoroutine(LoadNewScene()); //here is my error
    }

    IEnumerator LoadNewScene() // my coroutine
    {
        yield return new WaitForSeconds(3);

        AsyncOperation async = SceneManager.LoadSceneAsync(scene);

        while (!async.isDone)
        {
            yield return null;
        }
    }
}