How can I find all the doors getting null exception?

Question

The first script that control the whole doors: In this script I'm calling:

LockDoor(i);

Just for testing locking all the doors.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class DoorsManager : MonoBehaviour
{
    private List<HoriDoorManager> _doors = new List<HoriDoorManager>();

    private void Start()
    {
        var doors = GameObject.FindGameObjectsWithTag("Door");
        foreach(var door in doors)
        {
            _doors.Add(door.GetComponent<HoriDoorManager>());
        }

        for(int i = 0; i < _doors.Count; i++)
        {
            LockDoor(i);
        }

        // find all your doors and add them to the _doors collection
        // alternatively expose the collection to the UI and build it there
    }

    public void LockDoor(int doorIndex)
    {
        _doors[doorIndex].ChangeLockState(true);
    }
    public void UnlockDoor(int doorIndex)
    {
        _doors[doorIndex].ChangeLockState(false);
    }
}

The second script that attached to each door. And each door is built with two children doors that open by sliding to the sides:

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class HoriDoorManager : MonoBehaviour
{
    private bool doorLockState;
    private List<DoorHori> doors = new List<DoorHori>();

    private void Start()
    {
        GameObject doorsParent = GameObject.Find("Wall_Door_Long_01");
        foreach(Transform door in doorsParent.transform)
        {
            doors.Add(door.GetComponent<DoorHori>());
        } 
    }

    void OnTriggerEnter()
    {
        if (doorLockState == false)
        {
            if (doors != null)
            {
               for(int i =0; i < doors.Count; i++)
                {
                    doors[i].OpenDoor();
                }
            }
        }
    }

    public void ChangeLockState(bool lockState)
    {
        doorLockState = lockState;
    }
}

In the Start when it's adding the doors to the List all the doors are null:

GameObject doorsParent = GameObject.Find("Wall_Door_Long_01");
            foreach(Transform door in doorsParent.transform)
            {
                doors.Add(door.GetComponent<DoorHori>());
            } 

A screenshot of a door structure and the doors I'm trying to get in the HoriDoorManager script are the Door_Left and Door_Right

Why it's null ? Should I make instance new instance for the List of doors on OnTriggerExit ? Comparing: doorLockState = lockState; is logic ?

When running the game all the doors are locked but I'm getting the null exception.

I solved the first problem:

Since the script is attached to a child first I'm checking if the child have a parent. Then getting the parent then searching for the specific "Wall_Door_Long_01" GameObject on this parent and then making a foreach over the "Wall_Door_Long_01" GameObject:

private void Start()
    {
        if(transform.parent != null)
        {
            Transform parent = transform.parent;
            var doorsParent = parent.Find("Wall_Door_Long_01");
            foreach(Transform door in doorsParent)
            {
                test.Add(door);
                doors.Add(door.GetComponent<DoorHori>());
            }
        }
    }

The result is that the List test contains two GameObjects Door_Left and Door_Right and the List doors contains the two scripts from each of the doors.

Now I don't need the test List but I do need the doors List to be able to call the OpenDoor method.