ASP.Net Error: Cannot insert the value NULL into column UserId, column does not allow nulls. UPDATE fails

Hi All,

I am facing error while updating the Database from a GridView:

Cannot insert the value NULL into column 'UserId', table 'QuizSoft.dbo.EXAMCategory'; column does not allow nulls. UPDATE fails. The statement has been terminated.

But after refreshing, the page, the where updated. Why do i get this error message each time. the following is my settings:

The error comes from UserId which is not null from the tables

<asp:TemplateField HeaderText="Action">  
    <ItemTemplate>  
        <asp:LinkButton ID="LkBtnEdit" runat="server" CommandName="Edit" CssClass="btn btn-primary">Edit</asp:LinkButton>  
        <asp:LinkButton ID="LkBtnDelete" runat="server" CommandName="Delete" CssClass="btn btn-danger" OnClientClick="return confirm('Are you sure you want to delete?')">Delete</asp:LinkButton>  
    </ItemTemplate>  
    <EditItemTemplate>  
        <asp:LinkButton ID="LkBtnUpdate" runat="server" CommandName="Update" CssClass="btn btn-success">Update</asp:LinkButton>  
        <asp:LinkButton ID="LkBtnCancel" runat="server" CommandName="Cancel" CssClass="btn btn-secondary">Cancel</asp:LinkButton>  
    </EditItemTemplate>  
</asp:TemplateField> 

protected void GridView1_UpdateRecord(object sender, GridViewUpdateEventArgs e)
{
    if (GridView1.Rows.Count > 0)
    {
        GridViewRow row = (GridViewRow)GridView1.Rows[e.RowIndex];

        int categoryId = Int32.Parse(GridView1.DataKeys[e.RowIndex].Value.ToString());

        TextBox tCategoryName = (TextBox)row.FindControl("txtCategoryName");
        FileUpload fFileUpload1 = (FileUpload)row.FindControl("FileUpload1");
        DropDownList ddCategoryIsActive = (DropDownList)row.FindControl("dropDownCategoryIsActive");
        TextBox tUserId = (TextBox)row.FindControl("txtUserId");

        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionQuiz"].ToString());
        SqlCommand cmd = new SqlCommand();


        string userIdValue = GridView1.Rows[e.RowIndex].Cells[4].Text;

        string path = "CategoryImages/";

        if (fFileUpload1.HasFile)
        {
            path += fFileUpload1.FileName;

            //save image in folder    
            fFileUpload1.SaveAs(MapPath(path));
        }
        else
        {
            // use previous user image if new image is not changed    
            Image img = (Image)GridView1.Rows[e.RowIndex].FindControl("imgCatPicPath");
            path = img.ImageUrl;
        }

        if (!string.IsNullOrEmpty(tUserId.Text))
        {
            try
            {
                con.Open();
                cmd.CommandText = "exam_UpdateCategory";
                cmd.CommandType = CommandType.StoredProcedure;

                cmd.Parameters.Add("@CategoryID", SqlDbType.Int).Value = categoryId;
                cmd.Parameters.Add("@CategoryName", SqlDbType.VarChar, 50).Value = tCategoryName.Text.Trim();
                cmd.Parameters.Add("@CategoryBilde", SqlDbType.NVarChar, 500).Value = path.Trim();
                cmd.Parameters.Add("@CategoryIsActive", SqlDbType.Int).Value = int.Parse(ddCategoryIsActive.SelectedValue);
                cmd.Parameters.Add("@UserId", SqlDbType.Int).Value = int.Parse(tUserId.Text.Trim());

                cmd.Connection = con;
                cmd.ExecuteNonQuery();

                con.Close();
                cmd.Dispose();

                GridView1.EditIndex = -1;
                GridView1.DataBind();
                lblResult.Text = "Category Updated Successfully";
                // ImageData();
            }
            catch (SqlException ee)
            {
                throw ee;
            }
        }
        else
        {
            lblResult.Text = "userid value was null";
        }
    }
}

How to solve this issue?

Many thanks in advance