need code for update button using stored procedure in hidden id

Question

hi 

i create stored procedure like below

ALTER procedure [dbo].[sampleregis]

@RegId int,

@FullName varchar(50),

@UserName varchar(50),

@Password varchar(50),

@RePassword varchar(50),

@Gender char(10),

@Age int,

@Address varchar(50),

@option varchar(10)

as

set nocount off

IF  @option='delete'

 

delete from samplereg1 where RegId=@RegId  

if @option='insert'

IF EXISTS(select FullName,UserName,Password,RePassword,Gender,Age,Address from samplereg1 where FullName=@FullName)

return -1

ELSE

insert into samplereg1 (FullName,UserName,Password,RePassword,Gender,Age,Address)values(@FullName,@UserName,@Password,@RePassword,@Gender,@Age,@Address) 

 

if @option='update'

begin

UPDATE samplereg1 set Fullname=@Fullname,UserName=@UserName,Password=@Password,RePassword=@RePassword,Gender=@Gender,Age=@Age,Address=@Address where RegId=@RegId 

End 

in that i want to make update using button.so i have to use regid as a hidden field.

in my update button code is like below

 TextBox txtRef1=FindControl("TxtName") as TextBox;

            TextBox txtVal2=FindControl("TxtUserName") as TextBox;

            TextBox txtRec3=FindControl("TxtPassword") as TextBox;

            TextBox txtRbv4=FindControl("TxtRePassword") as TextBox;

            TextBox txtRnh5=FindControl("DropDownList1") as TextBox;

            TextBox txtRds6=FindControl("TxtAge") as TextBox;

            TextBox txtRgr7=FindControl("TxtAddress") as TextBox;

            HiddenField hdnRef8=FindControl("hdnRefId") as HiddenField;

            SqlConnection conn = new SqlConnection(GetConnectionString());

            SqlCommand cmd = new SqlCommand("sampleregis", conn);

            cmd.CommandType = CommandType.StoredProcedure;

            cmd.Parameters.Add("@RegId", SqlDbType.Int).Value = Convert.ToInt32(hdnRef8.Value); 

            cmd.Parameters.Add("@FullName", SqlDbType.VarChar, 50).Value = txtRef1.Text;

            cmd.Parameters.Add("@UserName", SqlDbType.VarChar, 50).Value = txtVal2.Text;

            cmd.Parameters.Add("@Password", SqlDbType.VarChar, 50).Value = txtRec3.Text;

            cmd.Parameters.Add("@RePassword", SqlDbType.VarChar, 50).Value = txtRbv4.Text;

            cmd.Parameters.Add("@Gender", SqlDbType.VarChar, 50).Value = txtRnh5.Text;

            cmd.Parameters.Add("@Age", SqlDbType.Int).Value = Convert.ToInt32(txtRds6.Text);

            cmd.Parameters.Add("@Address", SqlDbType.VarChar, 50).Value = txtRgr7.Text;

            cmd.Parameters.Add("@option", SqlDbType.VarChar, 10).Value = "update";

            conn.Open();

            

            

            cmd.ExecuteNonQuery();

            conn.Close();

 but if i execuite means ill get error like this line 

cmd.Parameters.Add("@RegId", SqlDbType.Int).Value = Convert.ToInt32(hdnRef8.Value); 

error is 

Object reference not set to an instance of an object.

 so please any one help me .i need urgently.

 thanks in advance

HARI