I am getting the error in the title when I run this code during the `Add` method. The `Add` method should add the gameobject to a list called queue.  

`GameObject` is a class.  

`GameManager` is a class as well.  

`queue` is a list. 

I think this is the only code relevant.

```
    static void Main()
    {
        GameObject obj1 = new GameObject();
        GameManager manager1 = new GameManager();
        obj1.name = "First";
        manager1.Add(obj1);
        manager1.Process();
    }

    public void Add(GameObject gameObject)
    {
        gameObject.initialize = true;
        queue.Add(gameObject);
    }

```