Trying to get property of non-object pada codeigniter

Gan tolong stuck dengan message error

Severity: Notice

Message: Trying to get property 'kode_matakuliah' of non-object

Filename: administrator/nilai_daftar.php

Line Number: 9

Message: Trying to get property 'id_thn_akad' of non-object

Filename: administrator/nilai_daftar.php

Line Number: 10

view

<?php
$nilai = get_instance();
$nilai->load->model('krs_model');
$nilai->load->model('mahasiswa_model');
$nilai->load->model('matakuliah_model');
$nilai->load->model('tahunakademik_model');

$krs = $nilai->krs_model->get_by_id($id_krs[0]);
$kode_matakuliah = $krs->kode_matakuliah; //<- LINE 9
$id_thn_akad = $krs->id_thn_akad;         //<- LINE 10
?>

<div class="container-fluid">
    <div class="alert alert-success">
        <i class="fas fa-university"></i> DAFTAR NILAI MAHASISWA
    </div>

    <center>
        <legend><strong>DAFTAR NILAI MAHASISWA</strong></legend>
        <table>
            <tr>
                <td>Kode Matakuliah</td>
                <td>: <?php echo $kode_matakuliah; ?></td>
            </tr>
        </table>
    </center>
</div>

controller

public function simpan_nilai()
    {
        $query = array();
        $id_krs = $_POST['id_krs'];
        $nilai = $_POST['nilai'];

        for ($i = 0; $i < sizeof($id_krs); $i++) {
            $this->db->set('nilai', $nilai[$i])->where('id_krs', $id_krs[$i])->update('krs');
        }

        $data = array(
            'id_krs' => $id_krs
        );

        $this->load->view('templates_administrator/header');
        $this->load->view('templates_administrator/sidebar');
        $this->load->view('administrator/nilai_daftar', $data);
        $this->load->view('templates_administrator/footer');
    }