I have this error a lot in 4.1 B1 & B2 with the MVC architecure.
Somehow, the ComponentQuery mechanism seems to have a bug in this function:

Code:
getDockedItems : function(selector, beforeBody) {
        var dockedItems = this.getComponentLayout().getDockedItems('render', beforeBody);

        if (selector && dockedItems.length) {
            dockedItems = Ext.ComponentQuery.query(selector, dockedItems);
        }

        return dockedItems;
    },
Somehow sometimes this function hist a component without the required function!!
I think that since a DockingContainer can be overriden with another ComponentLayout, it can be the problem ...