Error with LoadMask + NestedList

Hi,

I'm trying to show the loading mask while loading data for a nested list but keep hitting this error in Chrome:

Uncaught TypeError: Object [object Object] has no method 'isLoading'

This is the code:

Code:

---

Ext.regController('Forum', {
 
    // Board index action
    Boards: function()
    {
        // Set upthe loading mask
        this.loadMask = new Ext.LoadMask(Ext.getBody(), {store: App.ForumStore});
       
        // Show the list of boards
        if (!this.boardsView) {
            this.boardsView = this.render({
                xtype: 'ForumBoardsView',
            });
        }

...

---

Code:

---

App.ForumStore = new Ext.data.TreeStore({
    model: 'Forum',
    proxy: {
        type: 'ajax',
        url: 'ajax.php?get=forumlist',
        reader: {
            type: 'tree',
            root: 'forums'
        }
    }
});

App.views.ForumBoards = Ext.extend(Ext.NestedList, {

    title: 'Select board',
    store: App.ForumStore,
    displayField: 'name',
    getItemTextTpl: function() {
        return '{name} <p class="small">{description}</p>';
    }
   
});
Ext.reg('ForumBoardsView', App.views.ForumBoards);

---

Sorry if this is a stupid question, just started using Sencha Touch.

Thanks in advance for any help! :D