NullReferenceExpection: Object reference not set to an instance of an object error.

Hello. So i have been working on my script for movement for my character and untill recently, it was working just fine. here is the full error: NullReferenceException: Object reference not set to an instance of an object
PlayerMovement.FixedUpdate () (at Assets/Character/Scripts/PlayerMovement.cs:32)

Does anyone have any ideas on whats going on? Here is the script:

using UnityEngine;
using System.Collections;

public class PlayerMovement : MonoBehaviour {

    public float speed = 6f;
    Vector3 movement;
    Rigidbody playerrigidbody;
    bool IsFlying = false;
    public float mousesensetivity = 2f;

	void Start () {

        playerrigidbody = GetComponent<Rigidbody>();
  
	
	}
	
	// Update is called once per frame
	void FixedUpdate () {
        // rotation
        float h = Input.GetAxisRaw("Horizontal");
        float v = Input.GetAxisRaw("Vertical");
        move(h, v);

        float Xrot = Input.GetAxisRaw("Mouse X") * mousesensetivity;
        transform.Rotate (0, Xrot, 0);

        float Yrot = Input.GetAxisRaw("Mouse Y") * mousesensetivity;

        Camera.main.transform.Rotate(Yrot, 0, 0);

       Vector3 speed = new Vector3(h, 0, v);

       speed = transform.rotation * speed;

    }

    void move (float h, float v)
    {
        if (Input.GetKey(KeyCode.Space))
        {
            if (IsFlying == false)
            {
                IsFlying = true;
                GetComponent<Rigidbody>().velocity = new Vector3(0, 6, 0);

            }
        }
        Vector3 Speed = new Vector3(h, 0, v);

        Speed = transform.rotation * Speed / 6;


        movement.Set(h, 0f, v);

        playerrigidbody.MovePosition(transform.position + Speed);
    }

    void OnCollisionEnter(Collision col)
    {
        if (col.gameObject.name == "floor" || col.gameObject.name == "PlatformPrototype08x01x08" || col.gameObject.name == "StepsPrototype04x02x02" || col.gameObject.name == "PlatformPrototype02x01x02")
        {
            IsFlying = false;
        }
    }

}

oooohhhhhh, that was it. I didn’t tag it as a main camera. Thanks for the help.