NullReferenceException only at first run

Questions & Answers
1

This code works, but I get the following message:

“NullReferenceException: Object reference not set to an instance of an object
SoundManager.OnLevelFinishedLoading (Scene scene, LoadSceneMode mode)”

I only get it at first run (Yes, I have the gameobject attached in first scene).
Then in every other scene everything is fine, no error message.
Also even if I go back to the first scene I don’t get the message back.
It works but I am trying to understand why this happens only when the game loads for the first time.

public class SoundManager : MonoBehaviour {

    public AudioClip[] levelMusicChangeArray;
    private AudioSource audioSource;

    public static SoundManager Instance;

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

    private void Start()
    {
        audioSource = GetComponent<AudioSource>();
        audioSource.volume = PlayerPrefsManager.GetMasterVolume();
    }

    private void OnEnable()
    {
        SceneManager.sceneLoaded += OnLevelFinishedLoading;
    }
    
    private void OnDisable()
    {
        SceneManager.sceneLoaded -= OnLevelFinishedLoading;
    }

    private void OnLevelFinishedLoading(Scene scene, LoadSceneMode mode)
    {
        int level = scene.buildIndex;
        AudioClip thisLevelMusic = levelMusicChangeArray[level];

        try
        {
            if (thisLevelMusic)
            {
                audioSource.clip = thisLevelMusic;
                audioSource.loop = true;
                audioSource.Play();
            }
        }
        catch (NullReferenceException e)
        {
            Debug.LogException(e);
        }       
    }
    
}
2

that’s because OnEnable is called prior to start and in turn receives the OnLevelFinishedLoading call prior to start sich is itself called before Start’s are called. And since you use the singleton pattern, after the first NullReferenceException, it knows it’s reference and the problem won’t appear again.
as a fix and future recommendations, do GetComponent calls performed on itself in Awake.

3

u selected the prefabs?

         catch (NullReferenceException e)
         {
             Debug.LogException(e);
         }       //what this does?  I do not understand