how can I solve this error?

 I write this code for showing totalprice in a gridview footer.but i see this error.when this page loaded user input his quantity on textboxes and press a  button,after that can see the total price in gridview footer.here is my click event code:

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protected void Button1_Click(object sender, EventArgs e)
{
if (GridView1.Rows.Count > 0)
{
for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
{
TextBox qty = (TextBox)GridView1.Rows[i].FindControl("textbox2");
Label pr = (Label)GridView1.Rows[i].FindControl("label3");
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