Magento – Fatal error: Call to a member function toHtml() on a non-object

In Adminhtml (Sales->Order->view ) i have add the fieldset, this shows the content correctly.

When i am using the submitAndReloadArea the value have submited and saved successfully but the value retrieve i got the below error,

Fatal error: Call to a member function toHtml() on a non-object in /var/www/html/emsstore/app/code/local/Mage/Adminhtml/controllers/Sales/OrderController.php on line 783

Below are my files

i got error in $response = $this->getLayout()->getBlock('order_pickstatus')->toHtml(); this code which is in OrderController.php.

OrderController.php

class Mage_Adminhtml_Sales_OrderController extends Mage_Adminhtml_Controller_Action
{
    .....
    .....
    public function changePickstatusAction()
    {
        try {
            $id = $this->getRequest()->getParam('order_id');
            $order = Mage::getModel('sales/order')->load($id);
            $response = false;
            $data = $this->getRequest()->getPost('pickup_status');
            $order->setpickup_status($data);
            $order->save();
            $response = $this->getLayout()->getBlock('order_pickstatus')->toHtml();

        }
        catch (Mage_Core_Exception $e) {
            $response = array(
                'error'     => true,
                'message'   => $e->getMessage(),
            );
            $response = Mage::helper('core')->jsonEncode($response);
        }
        catch (Exception $e) {
            $response = array(
                'error'     => true,
                'message'   => $this->__('Cannot add order history.')
            );
            $response = Mage::helper('core')->jsonEncode($response);
        }
        $this->getResponse()->setBody($response);
    }
}

app/design/adminhtml/default/default/layout/sales.xml

<adminhtml_sales_order_changepickstatus>
    <block type="adminhtml/sales_order_view_pickstatus" name="order_pickstatus" template="sales/order/view/pickstatus.phtml" output="toHtml"></block>
</adminhtml_sales_order_changepickstatus>

<adminhtml_sales_order_view>
    .......
    <reference name="left">
        <block type="adminhtml/sales_order_view_tabs" name="sales_order_tabs">
            <block type="adminhtml/sales_order_view_tab_info" name="order_tab_info" template="sales/order/view/tab/info.phtml">
                .....
                <block type="adminhtml/sales_order_view_pickstatus" name="order_pickstatus" template="sales/order/view/pickstatus.phtml"></block>
                ...
        </block>
    </reference>
</adminhtml_sales_order_view>

template file,
app/design/adminhtml/default/default/template/sales/order/view/pickstatus.phtml

<div id="order_pickstatus_block">
    <div id="pickstatus_form" class="order-pickstatus-form">
        <span class="field-row">
            <label class="normal" for="pickup_status"><?php echo Mage::helper('sales')->__('Status') ?></label>
            <select name="pickup_status" id="pickup_status">
                <option value="">--Select Status--</option>
                <option value="notdelivered">Not Delivered</option>
                <option value="delivered">Delivered</option>
            </select>
        </span>
        <div class="f-right">
            <?php echo $this->getChildHtml('submit_button') ?>
        </div>
        <div class="clear"></div>
    </div>
    <div class="divider"></div>

</div>

Block file

class Mage_Adminhtml_Block_Sales_Order_View_Pickstatus extends Mage_Adminhtml_Block_Template
{
    protected function _prepareLayout()
    {
        $onclick = "submitAndReloadArea($('order_pickstatus_block').parentNode, '".$this->getSubmitUrl()."')";
        $button = $this->getLayout()->createBlock('adminhtml/widget_button')
            ->setData(array(
                'label'   => Mage::helper('sales')->__('Submit Status'),
                'class'   => 'save',
                'onclick' => $onclick
            ));
        $this->setChild('submit_button', $button);
        return parent::_prepareLayout();
    }

    public function getOrder()
    {
        return Mage::registry('sales_order');
    }

    public function getSubmitUrl()
    {
        return $this->getUrl('*/*/changePickstatus', array('order_id'=>$this->getOrder()->getId()));
    }
    public function getPickstatus()
    {
        return $this->getOrder()->getPickup_status();
    }



}

what is wrong in my code.

Anything more to config?

thanks in advance.