Render webcontrol to string

[Henrik Stidsen]

I need to get the HTML output from a webcontrol as a string instead of
it being rendered on the page.

I have the following code:
Dim SiteMapSource As New SiteMapDataSour ce()
Dim TreeView1 As New TreeView()
TreeView1.DataS ource = SiteMapSource
TreeView1.ShowE xpandCollapse = True
TreeView1.Expan dDepth = 1
TreeView1.DataB ind()
Dim sb As System.Text.Str ingBuilder = New System.Text.Str ingBuilder()
Using tw As System.IO.Strin gWriter = New System.IO.Strin gWriter(sb)
Using hw As System.Web.UI.H tmlTextWriter = New
System.Web.UI.H tmlTextWriter(t w)
TreeView1.Rende rControl(hw)
Response.Write( sb.ToString())
End Using
End Using

The line with TreeView1.Rende rControl(hw) throws a
NullReferenceEx ception with a almost unusable stacktrace:
[NullReferenceEx ception: Object reference not set to an instance of an
object.]
System.Web.UI.C ontrol.get_Spac erImageUrl() +59
System.Web.UI.W ebControls.Tree View.RenderBegi nTag(HtmlTextWr iter
writer) +146
System.Web.UI.W ebControls.WebC ontrol.Render(H tmlTextWriter writer)
+17
System.Web.UI.C ontrol.RenderCo ntrolInternal(H tmlTextWriter writer,
ControlAdapter adapter) +25
System.Web.UI.C ontrol.RenderCo ntrol(HtmlTextW riter writer,
ControlAdapter adapter) +121
System.Web.UI.C ontrol.RenderCo ntrol(HtmlTextW riter writer) +22
SiteMapTest.Pag e_Load(Object sender, EventArgs e) in
D:\TreeView.asp x.vb:25
System.Web.UI.C ontrol.OnLoad(E ventArgs e) +99
System.Web.UI.C ontrol.LoadRecu rsive() +47
System.Web.UI.P age.ProcessRequ estMain(Boolean
includeStagesBe foreAsyncPoint, Boolean includeStagesAf terAsyncPoint)
+1061

The weird part is that it works fine if I do
Page.Form.Contr ols.add(TreeVie w1) AND it works fine if I use a button
or even a Calendar control...

So, why does TreeView throw a NullReference when asked to render itself
?