unityでNullReferenceExceptionエラー

前提・実現したいこと

http://denshikousaku.net/unity3d-3d-shooting-part1
http://denshikousaku.net/unity3d-3d-shooting-part2
こちらのサイトで敵を攻撃する所まで作りましたが、攻撃はできるが倒れる動作ができないので解決策を教えて下さい。

発生している問題・エラーメッセージ

NullReferenceException: Object reference not set to an instance of an object
PlayerMovement.Turning () (at Assets/Scripts/Player/PlayerMovement.cs:43)
PlayerMovement.FixedUpdate () (at Assets/Scripts/Player/PlayerMovement.cs:28)

該当のソースコード

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class PlayerMovement : MonoBehaviour
{
public float speed = 6f;

Vector3 movement;
Animator anim;
Rigidbody playerRigidbody;
int floorMask;
float camRayLength = 100f;

void Awake()
{
    floorMask = LayerMask.GetMask("Floor");
    anim = GetComponent<Animator>();
    playerRigidbody = GetComponent<Rigidbody>();
}

void FixedUpdate()
{
    float h = Input.GetAxisRaw("Horizontal");
    float v = Input.GetAxisRaw("Vertical");

    Move(h, v);
    Turning();
    Animating(h, v);
}

void Move(float h, float v)
{
    movement.Set(h, 0, v);

    movement = movement.normalized * speed * Time.deltaTime;

    playerRigidbody.MovePosition(transform.position + movement);
}

void Turning()
{
    Ray camRay = Camera.main.ScreenPointToRay(Input.mousePosition);


    RaycastHit floorHit;

    if (Physics.Raycast(camRay, out floorHit, camRayLength, floorMask))
    {
        Vector3 playerToMouse = floorHit.point - transform.position;
        playerToMouse.y = 0f;

        Quaternion newRotation = Quaternion.LookRotation(playerToMouse);
        playerRigidbody.MoveRotation(newRotation);
    }
}

void Animating(float h, float v)
{
    bool walking = h != 0f || v != 0f;
    anim.SetBool("IsWalking", walking);
}