How to reference PivotTable control?

[Guest]

I have dragged a PivotTable(which id="PivotTable1") control on my
page(Index.aspx). And, How do I reference this PivotTable control in my
code-behind(index.aspx.vb)?

Because I notice when I drag the PivotTable from Toolbox to my web-page, I
can't find the line 'Protected WithEvents PivotTable1 As OWC10.PivotTable' as
other controls. So, I manually add this line in my code-behind, and get
PivotTable1.XMLData into a string, this web-page will prompt me the following
error:

Object reference not set to an instance of an object.

Please help, thanks.