Can anyone help me with getting data from the textbox template field in the gridview?

Here is my code for the textbox template field in the gridview.

<Columns>
<asp:TemplateField HeaderText="Quantity">
<ItemTemplate>
<asp:textbox runat="server" ID="QuantityTxtBox" />
</ItemTemplate>
</asp:TemplateField>
</Columns>

After entering data into the textbox in the gridview, I try to get the data in the first row and display it into LABEL1 with the following code

Dim text As String
Dim tb As TextBox = CType(LecReqGV.Rows(0).FindControl("QuantityTextbox"), TextBox)

If Not (tb.Text.Equals("")) Then
text = tb.Text
Label1.Text = text
End If


The code doesn’t work and the error message “Object reference not set to an instance of an object.” keeps prompting out. Can anyone help me with that?