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>