Value cannot be null. in C# WinApp

public partial class FRM_RPT_CAR_ID : Form
    {
        int x;

        SqlConnection con = new SqlConnection("Server= .; Database= WhiteWhaleDB; Integrated Security = true");
        SqlCommand sd;
        SqlDataReader sda;
        string s;

        public FRM_RPT_CAR_ID()
        {
            InitializeComponent();
        }

        public FRM_RPT_CAR_ID(string x)
        {
            InitializeComponent();

            this.x = int.Parse(x);
        }

        private void FRM_RPT_CAR_ID_Load(object sender, EventArgs e)
        {
            s = "select s1.Car_id, s1.Change_date , Service_Type.Servce_desc,s1.Current_dist, s1.Prev_distance as Prev_dist, (s1.Current_dist - s1.Prev_dist) as Change_dist ,s1.value_Tashhem as Value from Tashhem s1 INNER JOIN Service_Type ON s1.Servce_id = Service_Type.Servce_id where s1.Car_id" + x;
            DataSet1 ds = new DataSet1();
            SqlDataAdapter dataAdapter = new SqlDataAdapter(s, con);
            dataAdapter.Fill(ds.Tables["Car_details"]);
            CrystalReport_Car_id report = new CrystalReport_Car_id();
            report.SetDataSource(ds.Tables["Car_details"]);
            crystalReportViewer_Car_id.ReportSource = report;
            crystalReportViewer_Car_id.Refresh(); 
        }