The source code

    // Implementation class for created the tree powered form field
    ListSelector = Ext.extend(Ext.ux.TreeSelector, {
    	maxHeight:200,
    	listenForLoad: false,
        initComponent : function(){

    		this.tree = new Ext.tree.TreePanel({
    			animate:false,
    			border:false,
    			width: this.treeWidth || 180,
    			autoScroll:true,
    			useArrows:true,
    			selModel: new Ext.tree.ActivationModel(),
    			loader : new ListLoader({store: this.store})
    		});

    		var root = new Ext.tree.AsyncTreeNode({
    	        text: 'All Lists',
    			id: 'root',
    			leaf: false,
    			iconCls: 'icon-folder',
    			expanded: true,
    			isFolder: true
    	    });
    	    this.tree.setRootNode(root);

            this.tree.on('render', function(){
                this.store.bindTree(this.tree);
            }, this);

            ListSelector.superclass.initComponent.call(this);

    		// selecting folders is not allowed, so filter them
    		this.tree.getSelectionModel().on('beforeselect', this.beforeSelection, this);

    		// if being rendered before the store is loaded, reload when it is loaded
    		if(this.listenForLoad) {
    			this.store.on('load', function(){
    				root.reload();
    			}, this, {
    				single: true
    			});
    		}
        },

    	beforeSelection : function(tree, node){
    		if(node && node.attributes.isFolder){
    			node.toggle();
    			return false;
    		}
    	}
    });