I am a scripting beginner, and i wanted help in this script:

```
using UnityEngine;
using System.Collections;
public class RotateTurret : MonoBehaviour
{
// speed is the rate at which the object will rotate
public float speed;
void FixedUpdate ()
{
// Generate a plane that intersects the transform's position with an upwards normal.
Plane playerPlane = new Plane(Vector3.up, transform.position);
// Generate a ray from the cursor position
Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
// Determine the point where the cursor ray intersects the plane.
// This will be the point that the object must look towards to be looking at the mouse.
// Raycasting to a Plane object only gives us a distance, so we'll have to take the distance,
// then find the point along that ray that meets that distance. This will be the point
// to look at.
float hitdist = 0.0f;
// If the ray is parallel to the plane, Raycast will return false.
if (playerPlane.Raycast (ray, out hitdist))
{
// Get the point along the ray that hits the calculated distance.
Vector3 targetPoint = ray.GetPoint(hitdist);
// Determine the target rotation. This is the rotation if the transform looks at the target point.
Quaternion targetRotation = Quaternion.LookRotation(targetPoint - transform.position);
// Smoothly rotate towards the target point.
transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, speed * Time.deltaTime);
}
}
}
```

I found it in Unity’s official wikia (something like that). It does exactly what i want, that is rotate a turret towards the mouse in the Y axis. The problem is that i get this error:

“Object reference not setup to an instance of an object”

It’s in line 16 i think. It’s the ray ray part:

Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);