I am getting error while using count(*) query to count total numbers but i am getting error that ' Value can not be null parameter name dataTable'

Question

1. Here you can see I am using Count(*) query and I am getting error there is a picture with the code

private void List_Load(object sender, EventArgs e)
{

    try
    {
        if (Sqlcon.State == ConnectionState.Closed)
        {
            Sqlcon.Open();
        }
        string query = "Select Count(*) from AddCars ";
        SqlDataAdapter sdr = new SqlDataAdapter(query, Sqlcon);
        sdr.Fill(dt);
        totaltbl.Text = dt.Rows[0][0].ToString();



    }
    catch (Exception ex)
    {
        MessageBox.Show("Error" + ex);
    }

}