Error undefined variable upload file dengan php framework codeigniter

Saya mau membuat form dengan upload file menggunakan php framework CI tetapi saya menemui error seperti ini :

A PHP Error was encountered Severity: Notice

Message: Undefined variable: file

Filename: controllers/Bukusk.php

Line Number: 182

Backtrace:

File: C:\xampp\htdocs\siagsurat\application\controllers\Bukusk.php Line: 182 Function: _error_handler

File: C:\xampp\htdocs\siagsurat\index.php Line: 315 Function: require_once

A Database Error Occurred Error Number: 1048

Column 'file' cannot be null

codingan saya seperti ini : 1. Controller Bukusk.php

$this->_rules();

        if ($this->form_validation->run() == FALSE) {
                $this->create();
        }
        else
        {
            $config['upload_path']          = './uploads/bukusk';
            $config['allowed_types']        = 'pdf|jpg|jpeg|png';
            $config['max_size']             = 2048;

            $this->upload->initialize($config);
            if(!$this->upload->do_upload())
            {
                $no = $this->Bukusk_model->total_data()+1;
                $data = array(
                    'button' => 'Tambah',
                    'action' => site_url('bukusk/create_action'),
                    'navbar' => $this->navbar(),
                    'id_bukusk' => set_value('id_bukusk'),
                    'no' => set_value('no',$no),
                    'tgl_srt' => set_value('tgl_srt',date('d-m-Y')),
                    'no_sk1' => set_value('no_sk1',$no),
                    'no_sk2' => set_value('no_sk2'),
                    'no_sk3' => set_value('no_sk3',$this->romawi(date('m'))),
                    'no_sk4' => set_value('no_sk4',date('Y')),
                    'uraian' => set_value('uraian'),
                    'tahun' => set_value('tahun',date('Y')),
                    'menu' => 'Buku SK *Upload Error'.$this->input->post('berkas'),
                    'submenu' => 'Tambah Buku SK',
                    'backsub' => 'bukusk',
                    'lockmenu' => 'Tambah Buku SK',
                    'ui_sidebar_active' => 'Buku SK',
                    'isi' => 'bukusk/tb_bukusk_form',
                );
                    $this->load->view('template/wrapper', $data);
            }
            else
            {
                $data_berkas = $this->upload->data();
                $file = $data_berkas['file_name'];


            }

            $data = array(
                'no' => $this->input->post('no',TRUE),
                'tgl_srt' => date('Y/m/d',strtotime($this->input->post('tgl_srt',TRUE))),
                'no_sk1' => $this->input->post('no_sk1',TRUE),
                'no_sk2' => $this->input->post('no_sk2',TRUE),
                'no_sk3' => $this->input->post('no_sk3',TRUE),
                'no_sk4' => $this->input->post('no_sk4',TRUE),
                'uraian' => $this->input->post('uraian',TRUE),
                'tahun' => $this->input->post('tahun',TRUE),
                'file'  => $file,
                'id_user' => $this->session->userdata('id_user'),
                );

                $this->Bukusk_model->insert($data);
                $this->session->set_flashdata('message', 'Create Record Success');
                redirect(site_url('bukusk'));
}

2. View Form Input

<form enctype="multipart/form-data" action="<?php echo $action; ?>" method="POST">


	    <div class="form-group">
        <div class="row">
            <label for="int" class="col-sm-2 control-label">No <?php echo form_error('no') ?></label>
            <div class="col-sm-3">
                <input type="text" class="form-control" name="no" id="no" placeholder="No" value="<?php echo $no; ?>" readonly/>
            </div>
        </div>
        </div>
	    <div class="form-group">
        <div class="row">
            <label for="date" class="col-sm-2 control-label">Tanggal Surat <?php echo form_error('tgl_srt') ?></label>
            <div class="col-sm-3">
            <!--
                <input type="date" class="form-control" name="tgl_srt" id="tgl_srt" placeholder="Tgl Srt" value="<?php echo date('d/m/Y',strtotime($tgl_srt)); ?>" id="tanggal"/>
            -->
                <div class="input-group date">
                    <div class="input-group-addon">
                        <i class="fa fa-calendar"></i>
                    </div>
                      <input type="text" name="tgl_srt" class="form-control pull-right" id="tgl_srt" value="<?php echo date('d-m-Y',strtotime($tgl_srt)); ?>">
                </div>
            </div>
        </div>
        </div>

	    <div class="form-group">
        <div class="row">
            <label class="col-sm-2 control-label">Nomor SK <?php echo form_error('no_sk') ?></label>


            <div class="col-sm-1">
                <input type="text" class="form-control" name="no_sk1" id="no_sk1" placeholder="Nomor SK" value="<?php echo $no_sk1; ?>" readonly/>
            </div>
            <div class="col-sm-1">
                <center>/</center>
            </div>
            <div class="col-sm-2">
                <input type="text" class="form-control" name="no_sk2" id="no_sk2" placeholder="" value="<?php echo $no_sk2; ?>" />
            </div>
            <div class="col-sm-1">
            <center>/</center>
            </div>

            <div class="col-sm-2">
            <input type="text" class="form-control" name="no_sk3" id="no_sk3" placeholder="Nomor SK" value="<?php echo $no_sk3; ?>" readonly/>
            </div>
            <div class="col-sm-1">
            <center>/</center>
            </div>

            <div class="col-sm-2">
            <input type="text" class="form-control" name="no_sk4" id="no_sk4" placeholder="Nomor SK" value="<?php echo $no_sk4; ?>" readonly/>
            </div>
        </div>
        </div>


    <div class="form-group">
        <div class="row">
            <label for="varchar" class="col-sm-2 control-label">Uraian <?php echo form_error('uraian') ?></label>
            <div class="col-sm-6">
                <input type="text" class="form-control" name="uraian" id="uraian" placeholder="Uraian" value="<?php echo $uraian; ?>" />
            </div>
        </div>
        </div>

        <div class="form-group">
        <div class="row">
            <label for="varchar" class="col-sm-2 control-label">Tahun <?php echo form_error('tahun') ?></label>
            <div class="col-sm-3">
                <input type="text" class="form-control" name="tahun" id="tahun" placeholder="" value="<?php echo $tahun; ?>" readonly/>
            </div>
        </div>
        </div>

        <div class="form-group">
        <div class="row">
            <label for="varchar" class="col-sm-2 control-label">File</label>
            <div class="col-sm-3">
                <input type="file" class="form-control-file" name="berkas" />
            </div>
        </div>
        </div>



	    <input type="hidden" name="id_bukusk" value="<?php echo $id_bukusk; ?>" />
	    <button type="submit" class="btn btn-primary"><?php echo $button ?></button>
	    <a href="<?php echo site_url('bukusk') ?>" class="btn btn-default">Batal</a>
	</form>

saya juga sudah load library dan helper nya di autoload.php

$autoload['libraries'] = array('table', 'form_validation', 'database', 'session','simple_login','upload');

$autoload['helper'] = array('form','html','url');

Mohon bantuannya untuk pencerahannya, para suhu programmer :)