I need to add new tabpage in tabcontrol via user control C#

<pre> public SqlConnection cn = new SqlConnection(ConfigurationManager.ConnectionStrings["CustomersApp.Properties.Settings.CustomersConnectionString"].ConnectionString);

// Load data button in Form1 
   private void button1_Click(object sender, EventArgs e)
        {
            SqlCommand cm = new SqlCommand("select TOP(20) *from customers", cn);

            try
            {
                cn.Open();
                SqlDataAdapter da = new SqlDataAdapter(cm);
                DataTable dt = new DataTable();
                da.Fill(dt);
                foreach (DataRow dr in dt.Rows)
                {
                    customer_test1 pt = new customer_test1();
                    pt.UC_fullname = dr["customer_name"].ToString();
                    pt.UC_patientid = dr["customer_id"].ToString();
                   
                    flowLayoutPanel1.Controls.Add(pt);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

// code in user control with a button

Form1 ld;
 private void AddNewTab(string tabName)
        {
            if (InvokeRequired)
            {
                Invoke(new MethodInvoker(delegate { AddNewTab(tabName); }));
            }
            else
            {
                TabPage tp = new TabPage(tabName);
                ld.tabControl1.TabPages.Add(tp);
            }
        }

 private void button1_Click(object sender, EventArgs e)
        {
            AddNewTab("test1");
            
        }

By run the application and press the button of customer there is an error appear "System.NullReferenceException:'Objecte reference not set to an instance of an object'