Message: trying to get property 'id_produk' of non-object

Mohon bantuannya para master

Model:

// Gambar
    public function gambar($id_produk)
    {
        $this->db->select('*');
        $this->db->from('gambar');
        $this->db->where('id_produk', $id_produk);
        $this->db->order_by('id_gambar', 'desc');
        $query = $this->db->get();
        return $query->result();
    }

//Tambah gambar
    public function tambah_gambar($data)
    {
        $this->db->insert('gambar', $data);
    }

Controller:

// Gambar
    public function gambar($id_produk)
    {
        $produk         = $this->produk_model->detail($id_produk);
        $gambar         = $this->produk_model->gambar($id_produk);

        //Validasi input
        $valid = $this->form_validation;

        $valid->set_rules('judul_gambar','Judul/Nama Gambar','required',
            array(  'required'      => '%s harus diisi'));

        if($valid->run()) {
            $config['upload_path']      = './assets/upload/image';
            $config['allowed_types']    = 'gif|jpg|jpeg|png';
            $config['max_size']         = '2400';//Dalam KB
            $config['max-width']        = '2024';
            $config['max_height']       = '2024';

            $this->load->library('upload', $config);

            if( ! $this->upload->do_upload('gambar')){
        //Validasi End

        $data = array('title'       => 'TOOSWEET - Tambah Gambar Produk: '.$produk->nama_produk,
                      'produk'      => $produk,
                      'gambar'      => $gambar,
                      'error'       => $this->upload->display_errors(),
                      'isi'         => 'admin/produk/gambar'
                    );
        $this->load->view('admin/layout/wrapper', $data, FALSE);
        // Masuk database
        }else{
            $upload_gambar = array('upload_data' => $this->upload->data());

            // Create Thumbnails
            $config['image_library']    = 'gd2';
            $config['source_image']     = './assets/upload/image/'.$upload_gambar['upload_data']['file_name'];
            // Lokasi folder thumbnail
            $config['new_image']        = './assets/upload/image/thumbs/';
            $config['create_thumb']     = TRUE;
            $config['maintain_ratio']   = TRUE;
            $config['width']            = 250;
            $config['height']           = 250;
            $config['thumb_marker']     = '';

            $this->load->library('image_lib', $config);

            $this->image_lib->resize();
            // End Create Thumbnails

            $i = $this->input;

            $data = array(  'id_produk'         => $id_produk,
                            'judul_gambar'      => $i->post('judul_gambar'),
                            // Disimpan nama file gambar
                            'gambar'            => $upload_gambar['upload_data']['file_name'],
                        );
            $this->produk_model->tambah_gambar($data);
            $this->session->set_flashdata('Selamat', 'Data Gambar Telah Ditambahkan Ke Database');
            redirect(base_url('admin/produk/gambar/'.$id_produk),'refresh');
        }}
        // End masuk database
        $data = array('title'       => 'TOOSWEET - Tambah Gambar Produk',
                      'produk'      => $produk,
                      'gambar'      => $gambar,
                      'isi'         => 'admin/produk/gambar'
                    );
        $this->load->view('admin/layout/wrapper', $data, FALSE);
    }

View:

<?php
// Error upload
if(isset($error)) {
    echo '<p class="alert alert-warning">';
    echo $error;
    echo '</p>';
}

//Notifikasi error
echo validation_errors('<div class="alert alert-warning">','</div>');

//Form open
echo form_open_multipart(base_url('admin/produk/gambar/'.$produk->id_produk),' class="form-horizontal"');
?>

<div class="form-group">
    <label class="col-sm-2 control-label">Judul Gambar</label>
    <div class="col-sm-5">
    <input type="text" name="judul_gambar" class="form-control" placeholder="Judul Gambar Produk" value="<?php echo set_value('judul_gambar') ?>" required>
    </div>
</div>

<div class="form-group">
    <label class="col-sm-2 control-label">Upload Gambar</label>
    <div class="col-sm-5">
    <input type="file" name="gambar" class="form-control" placeholder="Upload Gambar Produk" value="<?php echo set_value('gambar') ?>" required>
    </div>
    <div class="col-md-2">
    <button class="btn btn-success btn-sm" name="submit" type="submit">
        <i class="fa fa-save"></i> Upload
    </button>
    <button class="btn btn-warning btn-sm" name="reset" type="reset">
        <i class="fa fa-times"></i> Reset
    </button>
    </div>
</div>

<?php echo form_close(); ?>

<?php
// Notifikasi
if($this->session->flashdata('sukses')) {
    echo '<p class="alert alert-success">';
    echo $this->session->flashdata('sukses');
    echo '</div>';
}
?>

<table class="table table-bordered" id="example1">
    <thead>
        <tr>
            <th>NO</th>
            <th>GAMBAR</th>
            <th>JUDUL</th>
            <th>ACTION</th>
        </tr>
    </thead>
    <tbody>
        <?php $no=1; foreach($gambar as $gambar) { ?>
        <tr>
            <td><?php echo $no ?></td>
            <td>
                <img src="<?php echo base_url('assets/upload/image/thumbs/'.$gambar->gambar) ?>" class="img img-responsive img thumbnail" width="60">
            </td>
            <td><?php echo $gambar->judul_gambar ?></td>
            <td>


                <a href="<?php echo base_url('admin/produk/delete_gambar/'.$produk->id_produk.'/'.$gambar->id_gambar) ?>" class="btn btn-danger btn-xs" onclick="return confirm('Yakin Ingin Menghapus Gambar Ini?')"><i class="fa fa-trash-o"></i> Hapus</a>


            </td>
        </tr>
        <?php $no++; } ?>
    </tbody>
</table>