MVC - reference to the viewport?

[steffenk]

There is a strange result with

this.getViewport()

it returns a reference to Ext.MessageBox ?!

The problem is that the reference is not available, but the viewport is the heart of the application.
I was not able for now to add/remove items in the viewport, even when i use itemId and ComponentQuery, for example

region: 'center',
itemId: 'CenterView',
items: [{
   //...
}]

i can get a reference by

var centerView = Ext.ComponentQuery.query('#CenterView');

but centerView.removeAll() throws an error.