Having trouble passing data through Lists

Question

What i wanted to do

I have a class (BlackBoard) that holdes data. and class NPC Gets the data from (BlackBoard). (NPC_Guard) inherits from (NPC) which uses (NPC_Movement) to move by using the data from (BlackBoard).

BlackBoard class

public sealed class BlackBoard : MonoBehaviour 
{
    #region Singleton
    private static BlackBoard instance = null;
    public static BlackBoard Instance
    {
        get
        {
            if (instance == null)
            {
                instance = new BlackBoard();
            }
            return instance;
        }
    }
    #endregion

    [SerializeField] List<Transform> NavigationPoints = new List<Transform>();
    public List<Transform> Get_NavigationPoints() => NavigationPoints;
    public void Set_NavigationPoints(List<Transform> T) => NavigationPoints = T;
}

NPC_Movement class

public class NPC_Movement
{

    Vector3 direction;

    List<Transform> _navigationPoints = null;
    [SerializeField] public List<Transform> NavigationPoints;
    {
        get
        {
            if (_navigationPoints == null)
            {
                _navigationPoints = BlackBoard.Get_NavigationPoints();
            }
            return _navigationPoints;
        }
    }
    BlackBoard BlackBoard = BlackBoard.Instance;
    

    public void Move()
    {
        // A complex movement function which uses NavigationPoints
    }
}

NPC class

public abstract class NPC : MonoBehaviour
{
    [SerializeField] protected NPC_Movement Movement;
}

NPC_Guard class

public class NPC_Guard : NPC
{
    void Update()
    {
        Movement.Move();
    }
}

Problem

NPC_Movement class does not get the NavigationPoints List .

What am i doing wrong

please help