Item.cs

[System.Serializable]
public class Item
{
	private string value = null;

	public void SetValue (string value) => this.value = value;
	
	public string GetValue () => value;
}

TestBehaviour.cs

using UnityEngine;

public class TestBehaviour : MonoBehaviour
{
    public Container Container;

    private void Start ()
    {
        // Expected to always log null.
        Debug.Log(Container.Items[0].GetValue());

        // This assignment should not persist after exiting-entering play mode,
        // as the value field has auto initializer set to null.
        Container.Items[0].SetValue("ASSIGNED AT PLAY MODE");
    }
}