Null Ref Exeption

Hi, I started a project then this error came from nowhere, please help me :( this is my code:

 private Transform firstPerson_View;
 private Transform firstPerson_Camera;
 private Vector3 firstPerson_View_Rotation = Vector3.zero;
 private float speed;
 private bool is_Moving, is_Grounded, is_Crouching;
 private float inputX, inputY;
 private float inputX_Set, inputY_Set;
 private float inputModifyFactor;
 private float antiBumpFactor = 0.75f;
 private bool limitDiagonalSpeed;
 private CharacterController charController;
 private Vector3 moveDirection = Vector3.zero;

 public float walkSpeed = 6.75f;
 public float runSpeed = 10f;
 public float crouchSpeed = 4f;
 public float jumpSpeed = 8f;
 public float gravity = 20f;

 void Start () 
 {
     firstPerson_View = transform.Find("FPS View");
     charController = GetComponent<CharacterController>();
     speed = walkSpeed;
     is_Moving = false;
 }
 
 void Update () 
 {
     PlayerMovement();
 }

 void PlayerMovement()
 {
     if(Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.S))
     {
         if(Input.GetKey(KeyCode.W)){inputY_Set = 1f;}
         else{inputY_Set = -1f;}
     }
     else{inputY_Set = 0f;}

     if(Input.GetKey(KeyCode.A) || Input.GetKey(KeyCode.D))
     {
         if(Input.GetKey(KeyCode.A)){inputX_Set = -1f;}
         else{inputX_Set = 1f;}
     }
     else{inputX_Set = 0f;}

     inputY = Mathf.Lerp(inputY, inputY_Set, Time.deltaTime * 19f);
     inputX = Mathf.Lerp(inputX, inputX_Set, Time.deltaTime * 19f);

     inputModifyFactor = Mathf.Lerp(inputModifyFactor, (inputY_Set != 0 && inputX_Set != 0 && limitDiagonalSpeed) ? 0.75f : 1.0f , Time.deltaTime * 19f);

     firstPerson_View_Rotation = Vector3.Lerp(firstPerson_View_Rotation, Vector3.zero, Time.deltaTime * 5f);
     firstPerson_View.localEulerAngles = firstPerson_View_Rotation;

     if(is_Grounded)
     {
         moveDirection = new Vector3(inputX * inputModifyFactor, -antiBumpFactor, inputY * inputModifyFactor);
         moveDirection = transform.TransformDirection(moveDirection) * speed;
     }

     moveDirection.y -= gravity * Time.deltaTime;

     is_Grounded = (charController.Move(moveDirection * Time.deltaTime) & CollisionFlags.Below) != 0;

     is_Moving = charController.velocity.magnitude > 0.15f;
 }

}

the error code line is: firstPerson_View.localEulerAngles = firstPerson_View_Rotation;