NullReferenceException when call an event

Hi everyone :)!

I have a countdown float that is used for a wavespawner:

    private float _countdown = 0;
	public delegate void OnCountdownDelegate();
	public event OnCountdownDelegate OnCountdownChange;

	public float countdown{
			return _countdown;
			if(value  < 0){
				_countdown = 0;
				_countdown = value;
			OnCountdownChange();//<--- The error says this does not exist yet (?

But when i start the game it says the following:

NullReferenceException: Object reference not set to an instance of an object
WaveSpawner.set_countdown (Single value) (at Assets/Scripts/Managers/WaveSpawner.cs:27)
WaveSpawner.Awake () (at Assets/Scripts/Managers/WaveSpawner.cs:39)

The error points the event i call when the countdown is setted.
And when this happens the script auto-disables himself. But when i enable the script manually everything goes fine!
I have this exact code but for money and no error appears.

Send help please!
P.D. Thanks.

Event is always null if no functions are subscribed to it. Just add a null check:

if (OnCountDownChange() != null)