How to Write DwtTree in main content zimlet tab

[itsky]

I'm a new member. I'm learning zimlet.
When I create a zimlet tab, I want to draw a tree in the content areas with code

com_zimbra_explorer_HandlerObject.prototype.appAct ive = function(appName, active){
switch(appName){
case this._extab:{
alert("appActive");
var app = appCtxt.getApp(this._extab);
var appToolbar = app.getToolbar();
appToolbar.setContent("<b>Dungnt</b>");
var shell = new DwtShell("MainShell");
var treeExplorer = new TreeExplorer(shell);
app.setContent(treeExplorer.innerHtml);
break;
};
}
};

with TreeExploer is

function TreeExplorer(parent) {
this._tree = new DwtTree(parent, null, null, DwtControl.ABSOLUTE_STYLE);

this._tree.setBounds(0, 0, Dwt.DEFAULT, "100%");

// Add the tree items to the tree
this._addItems();

// Add a selection listener to the tree. This listener will be called anytime a tree item is selected
this._tree.addSelectionListener(new AjxListener(this, this._treeListener));


// Make the tree visible. Specifically this moves the tree to the right "layer" (i.e. above the
// shell overlays)
this._tree.zShow(true);
}

But when I click on zimlet tab, It's have shown a warning Dialog with message is "DwtShell have exits for window"
How to declare a parent for DwtTree?
Thanks!