I Try To Populate Texbox On Selection Of Combobox Item But I Get An Error

public void LoadProduct()
       {
           strconn.Open();

           SqlDataAdapter da = new SqlDataAdapter("Select productID,productName from AS_Product", strconn);
           DataSet ds = new DataSet();
           da.Fill(ds);
           cmbproname.DataSource = ds.Tables[0];
           cmbproname.ValueMember = "productID";
           cmbproname.DisplayMember = "productName";
           strconn.Close();
       }

private void cmbproname_SelectedIndexChanged(object sender, EventArgs e)
     {
          string cm = "SELECT * FROM AS_Product WHERE productName='" + cmbproname.Text + "'";
         SqlDataAdapter da = new SqlDataAdapter(cm, strconn);
         SqlCommandBuilder cmd = new SqlCommandBuilder(da);
         DataSet ds = new DataSet();
         da.Fill(ds);
         if (cmbproname.SelectedIndex >=0)
         {
             txtmrp.Text = ds.Tables[0].Rows[0]["productMRP"].ToString();
         }
     }