NullReferenceException: Object reference not set to an instance of an object how to solve?

Question

3D

NullReferenceException: Object reference not set to an instance of an object How to solve?

I copied the entire script from the unity documentation
https://docs.unity3d.com/ru/530/ScriptReference/In...
Script:

public GameObject Ammo;
    void Update()
    {
        if (Input.GetButtonDown("Fire1"))
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            if (Physics.Raycast(ray))
                Instantiate(Ammo, transform.position, transform.rotation);
        }
    }

How to solve the error?

PS I want to shoot

Answer 1

Most likely, Ammo was not assigned to the inspector