Object reference not set to an instance of an object

I wanted to make my sprite rotate toward a cursor, but when I run my game, Unity is giving me this error at

"Vector3 mouseWorldSpace = Camera.main.ScreenToWorldPoint(mouseScreenPosition);". Help me, please!

That's my script:

public class PlayerMoving : MonoBehaviour {

 // Update is called once per frame
 void Update () {

     
     if (Input.GetKey (KeyCode.A)) 
         transform.Translate (new Vector3 (0, 5, 0) * Time.deltaTime);            
     
     if (Input.GetKey (KeyCode.D)) 
         transform.Translate (new Vector3 (0, -5, 0) * Time.deltaTime);
     
     if (Input.GetKey (KeyCode.W)) 
         transform.Translate (new Vector3 (5, 0, 0) * Time.deltaTime);
     
     if (Input.GetKey (KeyCode.S)) 
         transform.Translate (new Vector3 (-5, 0, 0) * Time.deltaTime);    

     Vector3 upAxis = new Vector3(0,0,-1);
     Vector3 mouseScreenPosition = Input.mousePosition;
     //set mouses z to your targets
     mouseScreenPosition.z = transform.position.z;
     Vector3 mouseWorldSpace = Camera.main.ScreenToWorldPoint(mouseScreenPosition);
     transform.LookAt(mouseWorldSpace, upAxis);
     //zero out all rotations except the axis I want
     transform.eulerAngles = new Vector3(0,0,-transform.eulerAngles.z);
 }

}