Alternative way to get a reference to the underlying Ext.Window element of Ext.MessageBox in ExtJS4
var msgBox = Ext.MessageBox.getDialog()

var w = Ext.MessageBox.confirm('Confirm', 'Are you sure you want to do that?', showResult);
w.close();

Ext.MessageBox.confirm('Confirm', 'Are you sure you want to do that?', showResult);
Ext.MessageBox.close();