Textbox inside Item Template Column not saving data

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VB

ASP.NET

I am saving grid data. Grid contains one textbox in item template column. But it was not saving any data, which was entered in textbox(Just saving 'NULL'). There was not error while debugging.

Any help is appreciated. Below is code i am using.

VB Code:

Public Sub btnsave_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnsave.Click
AddG()
End Sub
Public Sub AddG()
        For Each item As GridDataItem In GD_Prod.Items
Dim tx3 As TextBox = DirectCast(item("PL_STUDENT_Portions_Planned").FindControl("TxtSTUPort"), TextBox)
 Dim str3 As Object = tx3.text
            If tx3.Text = Nothing Then
                str3 = DBNull.Value
            ElseIf IsNumeric(tx3.Text) Then
                str3 = Val(tx3.Text)
            End If
Dim con As New SqlConnection(ConfigurationManager.ConnectionStrings("FConnectionString").ConnectionString)
            Dim cmd As New SqlCommand("P_FN_PR_InsertGridData", con)
            cmd.CommandType = CommandType.StoredProcedure
 cmd.Parameters.Add("@PL_Student_Portions_Planned", System.Data.SqlDbType.VarChar)
            cmd.Parameters("@PL_Student_Portions_Planned").Value = str3
con.Open()
            cmd.ExecuteNonQuery()
            con.Close()
Next
    End Sub

ASPX Code:

<telerik:RadGrid ID="GD_Prod" runat="server" AllowPaging="True" ShowHeader="False"
                    AutoGenerateColumns="False" GridLines="None" Height="173px" Skin="Outlook"
                    Width="1107px" PageSize="1">
                      <MasterTableView>
                              <Columns>
<telerik:GridTemplateColumn UniqueName="PL_STUDENT_Portions_Planned"
                               >
                                <ItemTemplate>
                                    <asp:TextBox ID="TxtSTUPort"   runat="server" Height="16px"
                                        Width="30px" ></asp:TextBox>
                                    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
                                        ControlToValidate="TxtSTUPort" ErrorMessage="*" ForeColor="#CC3300"
                                        SetFocusOnError="True" Display="Dynamic"></asp:RequiredFieldValidator>
                                    <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server"
                                        ControlToValidate="TxtSTUPort" ErrorMessage="Number" ForeColor="#CC3300"
                                        SetFocusOnError="True" ValidationExpression="[0-9]*" Display="Dynamic"
                                        Font-Size="6px"></asp:RegularExpressionValidator>
                                </ItemTemplate>
                                <HeaderStyle Width="19px" />
                            </telerik:GridTemplateColumn>
 </Columns>
                </telerik:RadGrid>
<asp:Button ID="btnsave" runat="server" Height="20px" Text="SAVE" Width="50px"
                        ToolTip="Click to Save " BackColor="#DFEFFF" ForeColor="#003399" />

Stored Proc:

ALTER PROCEDURE [DBO].P_FN_PR_InsertGridData
    (
@PL_Student_Portions_Planned varchar(10) = 0
)
AS
begin
INSERT INTO FNProdRecDetails (PL_Student_Portions_Planned)
values(@PL_Student_Portions_Planned)
End
Return