Array to string conversion on creating chained combobox with codeigniter - codeigniter

Question

i'm trying to create a chained combobox (2 combobox) with codeigniter, the result is ok but when i inspect it with dev tools on chrome show:
array to string conversion
and i don't know how to solve it.
Here's my code:
Controller:
public function create()
{
$this->data['get_all_data'] = $this->Category_model->getkat();
$this->load->view('back/tool/tool_add', $this->data);
}
function get_subkat()
{
$id_kat = $this->input->post('id_kat');
$datasubkat = $this->Category_model->getdatasubkat($id_kat);
$this->data .= "<option value=''>-- CHOOSE --</option>";
foreach($datasubkat as $data_subkat)
{
$this->data .= "<option value='$data_subkat[id_subkat]'>$data_subkat[nama_subkat]</option>";
}
echo $this->data;
}
Model:
function getkat()
{
$result = $this->db->get($this->table);
if($result->num_rows() > 0)
{
return $result->result_array();
}
else
{return array();}
}
function getdatasubkat($id_kat)
{
$this->db->where('id_kat', $id_kat);
$result = $this->db->get('subkategori');
if($result->num_rows() > 0)
{
return $result->result_array();
}
else
{return array();}
}
View:
<script type="text/javascript">
$(document).ready(function(){
$("#kat").change(function(){
var id_kat = $("#kat").val();
$.ajax({
type: 'POST',
url: "<?php echo base_url('admin/tool/get_subkat'); ?>",
data:"id_kat="+id_kat,
success: function(data) {
$("#subkat").html(data);
}
});
});
});
</script>
<div class="row">
<div class="col-xs-6"><b>Category</b>
<select name="kat" id="kat" class="form-control">
<option value="">-- CHOOSE --</option>
<?php
foreach($get_all_data as $category){
echo "<option value=".$category['id_kat'].">".$category['nama_kat']."</option>";
}
?>
</select>
</div>
<div class="col-xs-6"><b>Sub category</b>
<select name="subkat" id="subkat" class="form-control">
<option value="">-- CHOOSE --</option>
</select>
</div>
</div>
Any help will be so appreciated, thank you