code:

    public void FadeToLevel(int levelIndex) {
        levelToLoad = levelIndex;
        animator.SetTrigger("Fadeout");
        StartCoroutine(LoadScene());
    }

    public void OnFadeComplete() {
        finished = true;
    }

    IEnumerator LoadScene() {
        yield return null;

        AsyncOperation newLevel = SceneManager.LoadSceneAsync(levelToLoad);
        newLevel.allowSceneActivation = false;
        Debug.Log("loading new scene");
        while(!newLevel.isDone) {
            yield return null;

            if(finished && newLevel.progress >= 0.9f) {
                newLevel.allowSceneActivation = true;
            }
        }
    }