In Unity why cant I append this game object to an array. Error: ArgumentNullException: Value cannot be null. Parameter name: source [duplicate]

Here is the code being use:

private void start()
{
    for (int i = 0; i < 5; i++) 
    {
        GameObject card = Instantiate(deck[i], playersHandContent.transform);
        card.GetComponent<Button>().onClick.AddListener(delegate { AddCardToUse(card); });
    }
}

void AddCardToUse(GameObject cardSelected)
{
    Debug.Log(cardSelected.name);
    cardsSelected.Append(cardSelected);
}

I current calling a function when a card is clicked in the Unity UI. The card name is correct so it know its finding the object but when I attempt to append the GameObject to an array the error: "ArgumentNullException: Value cannot be null. Parameter name: source" appears. I am just trying to add the object to the array. Currently the system instantiates the object and adds a listener for the function below to add append it to an array, or atleast thats what I want it to do. I have no idea why im getting this issue as I've never encountered it in unity before.

SO far I have tried diffrent ways to get the GameObject though the button but all result in this out come. One version I tried was using this to get the object: UnityEngine.EventSystems.EventSystem.current.currentSelectedGameObject Even this way I still get the same reult

This also produced the same error: card = selectedCard; cardsSelected.Append(card); also give the same result