when is the Ext.msg.alert being called? The usual way within Ext.onready (i hope so).
otherwise i guess you are calling it before the html page has finished loading (thus the body is not available)
Please provide a full little testpage, the mistake should lie somewhere else than within the Message-function. Beside that i strongly recommend you use the current Ext 3.1.1. Many things have been fixed since Ext 3.0.0