Manually/dynamically load data into a treepanel?

Question

In ExtJs3, my team did the following to load data into a treestore. What's the equivalent way of doing this in ExtJS4? When I do similar code using a NodeInterface, I get a "Cannot read property '0' of null" error.

Ext.Ajax.request({
            url: servlet,
            success: function(response, opts)
            {
                var jsonData = Ext.decode(response.responseText);

                var root = new Ext.tree.AsyncTreeNode({
                    text: 'Root test',
                    draggable: false,
                    id: 'root',
                    children: jsonData.tree.data
                });
                tree.setRootNode(root);
//...

Update: I don't really care how to get this done. I just have two requirements: (1) use a single server request to get unrelated data. (2) reuse treepanels and gridpanels for different data

I've been able to load the data into dynamically-created treepanels. However, using setRootNode() doesn't work with existing tree panels.

//this works
var tree = Ext.create('Ext.tree.Panel', {
          root: {
                text:'Application',
                expanded:true,
                children: jsonResponse.categoryTree.data
          }
        });

//this doesn't work
existingTree.setRootNode(root);

Answer 1

So far the "good enough" solution is to

1. Turn the tree panel into a class

Each time the data changes:

1. Delete the previous instance (if any) of the tree panel
2. Instantiate a new tree panel

Not the "Ext JS 4" way but it gets the job done. Unless anyone can enlighten me with the correct way of doing things, I'm flagging this as the answer.

                var panel = Ext.ComponentQuery.query('panel#categorysearch')[0];

                //destroy the tree panel
                panel.remove('categorysearchtree', true);

                //create a new one
                var tree = Ext.create('MyApp.view.CategorySearchTreePanel', {
                    root: {
                        text:'Services', 
                        expanded:true,
                        children: jsonResponse.categoryTree.data
                    }
                });

                panel.add(tree);