Muncul pesan kesalahan : A PHP Error was encountered Severity: Notice Message: Undefined property:

public function  __construct()

    {

        parent::__construct();

        $this->load->model('mahasiswa_model');

        $this->load->library('form_validation');

    }

//VIEW (index)

<form action="" method="POST">

                <div class="input-group mt-1 mb-1">

                    <input type="text" class="form-control" name="keyword" placeholder="Cari....">

                    <div class="input-group-append">

                        <button class="btn btn-primary" type="submit">Cari</button>

                    </div>

                </div>

            </form>

//Controller (index)

public function index()

    {

        $data['judul'] = 'Halaman Mahasiswa';

        $data['mahasiswa'] = $this->mahasiswa_model->getallmahasiswa();

        if ($this->input->post('keyword')) {

            $data['mahasiswa'] = $this->Mahasiswa_model->cari();

        }

        $this->load->view('templates/header', $data);

        $this->load->view('mahasiswa/index', $data);

        $this->load->view('templates/footer');

    }

// Model ( cari() )

public function cari()

    {

        $keyword = $this->input->post('keyword', true);

        $this->db->like('nama', $keyword);

        return $this->db->get('tabel_mahasiswa')->result_array();

    }

//mohon solusi....masih belajar CI