At least for Magento 1.9 you have to be sure that extJs is loaded.
Use one of the following methods to activate the use of extJS in the backend:

a) In your controller use this:

$this->getLayout()->getBlock('head')->setCanLoadExtJs(true);

b) In your layout xml use this:

<reference name="head">
    <action method="setCanLoadExtJs">
        <value>1</value>
    </action>
</reference>

At least for Magento 1.9 you have to be sure that extJs is loaded.
Use one of the following methods to activate the use of extJS in the backend:

1. In your controller use this:

    $this->getLayout()->getBlock('head')->setCanLoadExtJs(true);
   

2. In your layout xml use this:

    <reference name="head">
        <action method="setCanLoadExtJs">
            <value>1</value>
        </action>
    </reference>