Object reference not set / Unable to cast object of type 'System.Web.UI.WebControls.Label' to type 'System.IConvertible'.

protected void LinkButton1_Click(object sender, EventArgs e)
        {
            try
            {
                int index = 0;

                LinkButton lb = (LinkButton) sender;
                GridViewRow row = (GridViewRow) lb.NamingContainer;
                if (row != null)
                {
                    index = row.RowIndex;
                }

                var labelId = (Label) NameDisplay.Rows[index].FindControl("labelGetId");
                labelId.Text = HiddenFieldID.Value;
                
                int empId = Convert.ToInt32(labelId.Text);
                using (var db = new knowitCVdbEntities())
                {

                    var theEmpl = (from
                                       p in db.EMPLOYEES
                                   where p.employee_id == empId
                                   select p).FirstOrDefault();

                    if (theEmpl != null)
                    {
                        theEmpl.firstname = TextBoxFirstName.Text;
                        theEmpl.lastname = TextBoxLastName.Text;


                    }

                }
            }
            catch (Exception x)
            {
                LabelError.Visible = true;
                LabelError.Text = x.Message;
            }
            

            }
        }
    }