Object reference not set to an instance of an object.

This is someting interesting but not figure out.
Basically, I use a FormView to view data in Database.
in the FooterTemplate, I have labels and a dropdownlist to show the record # of total records.
In the FormView, it has Update/New/Delete LinkButtons. I have not tried Update button, but tried other two buttons(New/Delete).
When I click the delete button, one record is deleted from the database and the databind correctly, in the footertemlate shows Record:#(dropdownlist) of #(total) changed
But if I click the New (change to Insert page) and then Insert button after I input some data.  it displays an error page with "Object reference not set to an instance of an object." But The record is added into the database actually however, it is not bind the data. I guess, when I delete one record, the # of records is decresed within the page.count. But Insert is a littl different, as it will add one index to the count. Here is my ...