Object reference not set to an instance of an object.

protected void ImageButton1_Click5(object sender, ImageClickEventArgs e)
    {
        int chkusers;
        string loginemailid, password;
        loginemailid = txtusername.Text.Trim();
        password = txtpw.Text.Trim();
        Session["username"] = loginemailid;
        Session["password"] = password;
        cn.Open();
        SqlCommand cmd = new SqlCommand("select loginemailid,password from tbl_register where loginemailid='" + txtusername.Text + "' and password='" + txtpw.Text + "'", cn);
        SqlDataReader dr = cmd.ExecuteReader();
        if (dr.HasRows)
        {
            while (dr.Read())
            {
                loginemailid = dr["loginemailid"].ToString();
                password = dr["password"].ToString();
                string userid = Request.QueryString["ids"];
                Session["userid"] = userid.ToString();
                Response.Redirect("Home.aspx?ids=" + userid);
            }
            dr.Close();
        }
        else 
        {
            lblmsg.Text = "Invalid Username and Password";
        }
      
    }