未处理NullReferenceException异常

  private void btnquery_Click(object sender, EventArgs e)
        {
            DBConnect();
            sqlcon.Open();//这里引发了一个异常NullReferenceException，初学者不怎么会，求大神
            SqlDataAdapter da = new SqlDataAdapter("Select ID as 学号," + "name as 姓名,class as 班级 from students WHERE ID " + "like '%" + cbID.Text + "%'", sqlcon);
            DataSet ds = new DataSet();
            da.Fill(ds, "tablename");
            dataGridView1.DataSource = ds.Tables[0];
            sqlcon.Close();
        }