Steps for reproduction:
1. Clean all the caches in the Explorer.
2. Close all Explorer and reopen one.
3. Try to edit an existing Web Content in Explorer-> the editor doesn't appear at first time because of the following error:
Webpage error details
User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0)
Timestamp: Thu, 16 Dec 2010 12:47:59 UTC
Message: 'length' is null or not an object
Then if you refresh the edit page the editor is shown correctly without this error.