Cant take a mouse position

I want to create a script what spawn a box in mouse position. But it cant take a mouse position because it sets to nul or somthing else.

An error is
NullReferenceException: Object reference not set to an instance of an object
Placement.Update () (at Assets/Scripts/Placement.cs:19)

public class Placement : MonoBehaviour
{
    Vector3 _MousePos;
    public GameObject _Objs;
    void Start()
    {
     
    }

    // Update is called once per frame
    void Update()
    {
        Vector3 _MousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition);

        if (Input.GetMouseButton(0))
        {
            
            Vector3 newmMousepos = new Vector3(_MousePos.x, _MousePos.y, _MousePos.z - 0.04f);
            Instantiate(_Objs, newmMousepos, Quaternion.identity);
        };
        
    }
}