NullReferenceException: Object reference not set to an instance of an object UnitClick.Update ()

Question

using UnityEngine;

public class UnitClick : MonoBehaviour
{
    private Camera myCam;

    public LayerMask clickable;
    public LayerMask ground;

    void Start()
    {
        myCam = Camera.main;
    }

    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            **Ray ray = myCam.ScreenPointToRay(Input.mousePosition);**
            RaycastHit hit;

            if (Physics.Raycast(ray, out hit, Mathf.Infinity, clickable))
            {
                if (Input.GetKey(KeyCode.LeftShift))
                {
                    UnitSelections.Instance.ShiftClickSelect(hit.collider.gameObject);
                }
                else
                {
                    UnitSelections.Instance.ClickSelect(hit.collider.gameObject);
                }
            }
            else
            {
                if (!Input.GetKey(KeyCode.LeftShift))
                {
                    UnitSelections.Instance.DeselectAll();
                }
            }
        }
    }
}

I killed more than half a day to find the source of the problem and did not find it...

I tried to rewrite a little and change the cameras to public ones and changed the raycast, but nothing helped...