Object reference not set to an instance of an object

Unity Engine
1

I made this code to can play music on start, but i get this error, and in other codes i put the same thing and there is working, why here is not working?

using UnityEngine;
using System.Collections;

public class PlayMusicC : MonoBehaviour
{
    private AudioManager audioManager;

    void Start()
    {
        audioManager = AudioManager.instance;

        if (audioManager == null)
        {
            Debug.LogError("PANIC! No audiomanager in scene.");
        }



        audioManager.PlaySound("DesertMusic");
    }
}

AudioManager Code:

using UnityEngine;
using UnityEngine.Audio;

[System.Serializable]
public class Sound
{

    public string name;
    public AudioClip clip;
    public AudioMixerGroup audioMixer;

    [Range(0f, 1f)]
    public float volume = 0.7f;
    [Range(0.5f, 1.5f)]
    public float pitch = 1f;

    [Range(0f, 0.5f)]
    public float randomVolume = 0.1f;
    [Range(0f, 0.5f)]
    public float randomPitch = 0.1f;

    public bool loop = false;

    private AudioSource source;

    public void SetSource(AudioSource _source)
    {
        source = _source;
        source.clip = clip;
        source.loop = loop;
        source.outputAudioMixerGroup = audioMixer;
    }

    public void Play()
    {
        source.volume = volume * (1 + Random.Range(-randomVolume / 2f, randomVolume / 2f));
        source.pitch = pitch * (1 + Random.Range(-randomPitch / 2f, randomPitch / 2f));
        source.Play();
    }

    public void Stop()
    {
        source.Stop();
    }

}

public class AudioManager : MonoBehaviour
{

    public static AudioManager instance;

    [SerializeField]
    Sound[] sounds;

    void Awake()
    {
        if (instance != null)
        {
            if (instance != this)
            {
                Destroy(this.gameObject);
            }
        }
        else
        {
            instance = this;
            DontDestroyOnLoad(this);
        }
    }

    void Start()
    {
        for (int i = 0; i < sounds.Length; i++)
        {
            GameObject _go = new GameObject("Sound_" + i + "_" + sounds[i].name);
            _go.transform.SetParent(this.transform);
            sounds[i].SetSource(_go.AddComponent<AudioSource>());
        }
    }

    public void PlaySound(string _name)
    {
        for (int i = 0; i < sounds.Length; i++)
        {
            if (sounds[i].name == _name)
            {
                sounds[i].Play();
                return;
            }
        }

        Debug.LogWarning("AudioManager: Sound not found in list, " + _name);
    }

    public void StopSound(string _name)
    {
        for (int i = 0; i < sounds.Length; i++)
        {
            if (sounds[i].name == _name)
            {
                sounds[i].Stop();
                return;
            }
        }

        Debug.LogWarning("AudioManager: Sound not found in list, " + _name);
    }

}
2

SOLVED!