Rotate to mouse direction

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);