Populate DataTable

Hi everyone, I'm having trouble populating a DataTable with a database.

[code]
public DataTable GetData()
    {
        string connectionString =
          System.Web.HttpContext.Current.Application["solarsearch_dbConnectionString"].ToString();

        SqlConnection commandString =
            new SqlConnection(connectionString);

        commandString.Open();

        string SQL = "select * from enquiry";

        SqlCommand objCommand = new SqlCommand(SQL, commandString);
        SqlDataAdapter adapter = new SqlDataAdapter();
        DataSet DS = new DataSet();
        adapter.SelectCommand = objCommand;
        adapter.Fill(DS);

        DataTable dt = DS.Tables[0];
        return dt;
    }
[/code]

When I execute the code I get the error by
[code]
string connectionString =
          System.Web.HttpContext.Current.Application["solarsearch_dbConnectionString"].ToString();
[/code]

saying Object reference not set to an instance of an object.
Any help please.