"Invalid attempt to read when no data is present" when using SQLDataReader

Here is my GeneralFunctions:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

/// <summary>
/// Summary description for GeneralFunctions
/// </summary>
public class GeneralFunctions
{
    public GeneralFunctions ()
    {
        //
        // TODO: Add constructor logic here
        //
    }

    public static DataTable GetData ( string query )
    {
        SqlDataAdapter dataAdapter;
        DataTable table;

        try
        {
            dataAdapter = new SqlDataAdapter( query, GetConnectionString() );
            table = new DataTable();

            dataAdapter.Fill( table );
            return table;
        }
        catch ( Exception ex )
        {
        }
        finally
        {
            dataAdapter = null;
            table = null;
        }

        return table;
    }

    private static string GetConnectionString ()
    {
        string connectionString = ConfigurationManager.ConnectionStrings[ "CAPortalConnectionString" ].ConnectionString;

        return connectionString;
    }

    public static int? AuthenticateLogin ( string username, string password )
    {
        using ( var conn = new SqlConnection( GetConnectionString() ) )
        using ( var cmd = conn.CreateCommand() )
        {
            conn.Open();
            cmd.CommandText =
            @"SELECT 
                 DistID 
             FROM 
                 Distributor
             WHERE 
                 Username = @username 
             AND 
                 Password = @password";
            cmd.Parameters.AddWithValue( "@username", username );
            cmd.Parameters.AddWithValue( "@password", password );
            using ( var reader = cmd.ExecuteReader() )
            {
                if ( !reader.Read() )
                {
                    // no results found
                    return null;
                }