The ConnectionString property has not been initialized random error

Hi,

Can someone please help me?

I randomly get an error when running my website:

The ConnectionString property has not been initialized.

First time I run the web page it works. Second time it fails. Is this got anything to do with static methods?

This is my code in DAL layer:

ConStrings.cs

public   class ConStrings
    {
        
        /// </summary>
        public  static    string PizzaDeliveryConnectionString
        {
            get
            {
                
                {
                    // get from PL web.config
                    string myconnectionString = ConfigurationManager.ConnectionStrings["PizzaDeliveryConnectionString"].ConnectionString;
                    return myconnectionString;
                   
                }
                            
            }
              
               }
 
        public static   SqlConnection GetPizzaDeliveryConnection()
        {
            return new SqlConnection(PizzaDeliveryConnectionString);
        }

DLCustomer.cs

 public  class DLCustomer
    {
     
         static SqlConnection connString = DL.ConStrings.GetPizzaDeliveryConnection();
 
         public static DataSet GetCustomerByPhoneNumber(string phonenumber)//static is good for returing data from db
         {
             DataSet ds = null;
             ds = new DataSet();
             try
             {
                 using (var connection = connString)//using doesn't require connection closed
                 {

                     connection.Open();
                      
                     using (SqlCommand cmd = new SqlCommand("GetCustomerByPhoneNumber", connection))
                     {

                         cmd.Connection = connection;

                         cmd.CommandType = System.Data.CommandType.StoredProcedure;
                         cmd.Parameters.AddWithValue("@phonenumber", phonenumber);

                         using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                         {

                             da.Fill(ds, "CustomersTable");
                         }
                     }

                        return ds;

                 }
             }

             catch (SqlException ex)
             {
                 return ds;
             }

         }

BUSINESS LAYER

Customer.cs

 public class Customer
    {
       
        private string phonenumber;
         private DL.DLCustomer customerData;
        public int CustomerId { get; set; }
        public string CustomerName { get; set; }
 

        public String PhoneNumber
        {
            get
            {
                return this.phonenumber;
            }
            set
            {
                this.phonenumber = value;
                if (this.PhoneNumber == "")
                {
                    throw new Exception("Please provide Tel ...");
                }

            }
        }

        public Customer ()  // Default Constructor
        {
            //An instance of the Data access layer!
            customerData = new DL.DLCustomer();
        }
        /// Function Find customer. Calls the
        /// function in Data layer.
        /// It returns the details of the customer using
        /// customer ID via a Dataset to GUI tier.
        /// </SUMMARY>

        public static  DataSet GetCustomerByPhoneNumber(Customer customer)
        {
            if (customer.phonenumber == "")
                throw new Exception("Please provide phonenumber to search");

             DataSet data = null;

             data = DL.DLCustomer.GetCustomerByPhoneNumber(customer.phonenumber);

            return data;
        }
    }

Presentation layer

 protected void btnOrder_Click(object sender, EventArgs e)
        {
            Customer customer = new Customer();
            
            //string dt = Request.Form[txtDate.UniqueID];
            customer.PhoneNumber = txtPhoneNumber.Text;
           // ClassLibrary1.DbFunc1.GetCustomerByPhoneNumber(customer) ;

            rptCustomers.DataSource = Customer.GetCustomerByPhoneNumber(customer);
            rptCustomers.DataBind();
        }

web.config

<configuration>
  <connectionStrings>
         <add name="PizzaDeliveryConnectionString" connectionString="Data Source=mycomp;Initial Catalog=PizzaDelivery;Integrated Security=True"      providerName="System.Data.SqlClient" />
  </connectionStrings>