Undefined property: U::$form_validation

Mohon bantuannya mastah.. ini kalimat lengkap erornya..

//////////////////////////////////

 A PHP Error was encountered

Severity: Notice

Message: Undefined property: U::$form_validation

Filename: controllers/U.php

Line Number: 28

Backtrace:

File: D:\xampp\htdocs\karyaid\application\controllers\U.php
Line: 28
Function: _error_handler

File: D:\xampp\htdocs\karyaid\index.php
Line: 315
Function: require_once

////////////////////////////////////
An uncaught Exception was encountered

Type: Error

Message: Call to a member function set_rules() on null

Filename: D:\xampp\htdocs\karyaid\application\controllers\U.php

Line Number: 28

Backtrace:

File: D:\xampp\htdocs\karyaid\index.php
Line: 315
Function: require_once

Nah masalahnya kan di kontroller U, ini isi filenya.

 <?php
defined('BASEPATH') OR exit('No direct script access allowed');

class U extends CI_Controller {

	public function __construct(){
		parent::__construct();
		$this->load->model('artikel_model');
	}

	///halaman dasbor
	public function index()
	{
		$artikel = $this->artikel_model->get_artikel();
		$data = array(
						'artikel' => $artikel,
						'isi' => 'u/index'
					);
		$this->load->view('layout/wrapper', $data);
	}

	public function post(){
		$this->load->helper('form');
		$this->load->library('form_validation');

		$this->form_validation->set_rules('judul_artikel', 'Judul', 'trim|required|min_length[5]|max_length[12]');
		$this->form_validation->set_rules('isi_artikel', 'Artikel', 'trim|required|min_length[5]');
		$this->form_validation->set_rules('status_artikel', 'Status', 'trim|required|min_length[5]|max_length[12]');
		$this->form_validation->set_rules('jenis_artikel', 'Jenis', 'trim|required|min_length[5]|max_length[12]');
		$this->form_validation->set_rules('keywords', 'Keywords', 'trim|required|min_length[5]');

		if($this->form_validation->run() === FALSE){
			$data = array(
							'isi' => 'u/post'
						);
			$this->load->view('layout/wrapper', $data);
		}else{
			$this->artikel_model->set_artikel();
			redirect('u/');
		}
	}

	public function postingan(){
		$data = array(
						'isi' => 'u/postingan'
					);
		$this->load->view('layout/wrapper', $data);
	}

}

/* End of file U.php */
/* Location: ./application/controllers/U.php */

Bantuin ngatasin masalah ini mastah, saya bener-bener gk nemu solusinya. Ini saya juga belajar perograman otodidak, jadi masih banyak kebingungan dan bug. Jd mohon bantuannya buat belajar