title: Javascript error: "Result of expression 'el' [null] is not an object" while adding any widget to CMS content
body: <p>In my local dev environment, any time I try to add a widget to a CMS page I get a javascript error: Result of expression 'el' [null] is not an object.</p>
  
  <p>Does anyone know what might cause that?</p>
  
  <p>Magento 1.5.1
  Mac OS X 10.6.7
  Apache 2.2.17
  Safari 5.0.5 / Firefox 3.5.9</p>
  
  <p>In Chrome 12.0.742.100, the error is:
  The page at localhost.com says:
  Cannot read property 'selectionStart' of null</p>
  
  <p>Has anyone seen this before?</p>