tidak bisa reload datatable setelah add data

permisi saya mau tanya,kenapa data yang saya masukan lewat form popup setelah di save malah muncul alert error tapi sebenarnya data sudah masuk

ini codenya:

function reload_table()
{
    table.ajax.reload(null,false); //reload datatable ajax
}

function save()
{
    $('#btnSave').text('saving...'); //change button text
    $('#btnSave').attr('disabled',true); //set button disable
    var url;

    if(save_method == 'add') {
        url = "<?php echo site_url('activity/save')?>";
    } else {
        url = "<?php echo site_url('activity/ajax_update')?>";
    }

    // ajax adding data to database
    $.ajax({
        url : url,
        type: "POST",
        data: $('#form-input').serialize(),
        dataType: "JSON",
        success: function(data)
        {

            if(data.status) //if success close modal and reload ajax table
            {
                $('#myModal').modal('hide');
                reload_table();

            }

            $('#btnSave').text('save'); //change button text
            $('#btnSave').attr('disabled',false); //set button enable

        },
        error: function (jqXHR, textStatus, errorThrown)
        {
            alert('Error adding / update data');
            $('#btnSave').text('save'); //change button text
            $('#btnSave').attr('disabled',false); //set button enable

        }
    });
}

my controller:

public function save() {
        $actype     	= $this->input->post('actype');
        $activity_name 	= $this->input->post('activity_name');
        $project  		= $this->input->post('project');
        $portion  		= $this->input->post('portion');
        $activity  		= $this->input->post('actid');

        $data = array(
        	'activity_type_id'	=>$actype,
			'activity_name'		=>$activity_name,
			'project_id'		=>$project,
			'portion' 			=>$portion,
			'activity_id' 		=> $activity

        );
        $this->activity->insertactivity($data);

        echo json_encode(array("status" => TRUE));
    }