combobox จังหวัด อำเภอ ตำบล แล้วมันขึ้น Object reference not set to an instance of an object.

string amphurid = cboCusamphur.SelectedValue == null ? "" : cboCusamphur.SelectedValue;

private void cboCusprovince_SelectedIndexChanged(object sender, EventArgs e)
        {
            string provinceid = cboCusprovince.SelectedValue.ToString();
            try
            {
                DBConnect db = new DBConnect();
                string sql = " select * from amphur where PROVINCE_ID=?province_id ";
                MySqlCommand SelectCommand = new MySqlCommand(sql, db.connection);
                db.OpenConnection();
                SelectCommand.Parameters.Add(new MySqlParameter("?province_id", provinceid));
                MySqlDataAdapter dataadapter = new MySqlDataAdapter(SelectCommand);
                dtamphur.Clear();
                dataadapter.Fill(dtamphur);
                if (dtamphur.Rows.Count > 0)
                {
                    cboCusamphur.DisplayMember = "AMPHUR_NAME";  
                    cboCusamphur.ValueMember = "AMPHUR_ID"; 
                    cboCusamphur.DataSource = dtamphur;
                    cboCusamphur.DataBind(); // ลืม Bind Data รึเปล่าครับ cboCusamphur ถึงมีค่าเป็น Null
                }
                db.CloseConnection();
            }
            catch (Exception ex)
            {

                MessageBox.Show(ex.Message);
            }
        }

        private void cboCusamphur_SelectedIndexChanged(object sender, EventArgs e)
        {
// เออเรอตรงบรรทัดนี้ครับ  Object reference not set to an instance of an object.
    string amphurid = cboCusamphur.SelectedValue == null ? "" : cboCusamphur.SelectedValue;
             try
            {                
                DBConnect db = new DBConnect();
                string sql = " select * from district where AMPHUR_ID=?amphurid";
                MySqlCommand SelectCommand = new MySqlCommand(sql, db.connection);
                db.OpenConnection();
                SelectCommand.Parameters.Add(new MySqlParameter("?amphurid", amphurid));
                MySqlDataAdapter dataadapter = new MySqlDataAdapter(SelectCommand);
                dtdistrict.Clear(); 
              
                dataadapter.Fill(dtdistrict);
               
                if (dtdistrict.Rows.Count > 0)
                {
                    cboCusdistrict.DisplayMember = "DISTRICT_NAME";  
                    cboCusdistrict.ValueMember = "DISTRICT_ID";  
                    cboCusdistrict.DataSource = dtdistrict;
                    cboCusdistrict.DataBind(); 
                }
                 db.CloseConnection();

            }
            catch (Exception ex)
            {

                MessageBox.Show(ex.Message);
            }

        }