title: Exceptions: System.NullReferenceException while trying to create relation between tables in dataSet
body: <p>using C#, .net framework 4.5, VS 2012</p>
  
  <p>Try to create simple relation between tables in data set, but got System.NullReferenceException, as I can see on MSDN, it's mean <code>occurs when you try to reference an object in your code that does not exist</code>. But, think i create all required objects.</p>
  
  <p>My code below:</p>
  
  <pre><code>    //create place for storing all tables from data base
      private DataSet myDS = new DataSet("AutoLot");
  
      //command builders for easy way access to tables
      private SqlCommandBuilder sqlCInventory;
      private SqlCommandBuilder sqlCOrders;
      private SqlCommandBuilder sqlCCustomers;
  
      //adapters for each table
      private SqlDataAdapter sqlAInventory;
      private SqlDataAdapter sqlAOrders;
      private SqlDataAdapter sqlACustomers;
  
      //connection string
      private string cnStr = string.Empty;
  
      public MainForm()
      {
          InitializeComponent();
          //get connection string from .config file
          cnStr = 
              ConfigurationManager.ConnectionStrings["AutoLotSqlProvider"].ConnectionString;
          //create adapters
          sqlACustomers = new SqlDataAdapter("Select * From Customers", cnStr);
          sqlAInventory = new SqlDataAdapter("Select * From Inventory", cnStr);
          sqlAOrders = new SqlDataAdapter("Select * From Orders", cnStr);
  
          //automatic generate commands 
          sqlCCustomers = new SqlCommandBuilder(sqlACustomers);
          sqlCInventory = new SqlCommandBuilder(sqlAInventory);
          sqlCOrders = new SqlCommandBuilder(sqlAOrders);
  
          //add table to data Set
          sqlAInventory.Fill(myDS);
          sqlAOrders.Fill(myDS);
          sqlACustomers.Fill(myDS);
  
          //create relationship between tables
          BuildTableRelationShip();
  
          //create DataSourse for datGrids on UI
          dataGridViewCustomer.DataSource = myDS.Tables["Inventory"];
          dataGridViewOrders.DataSource = myDS.Tables["Orders"];
          dataGridViewCustomer.DataSource = myDS.Tables["Customers"];
      }
  </code></pre>
  
  <p>and here  I got exception</p>
  
  <pre><code>    private void BuildTableRelationShip()
      {
          //create object of relationShips
          DataRelation dr = new DataRelation("CustomersOrders", //name of relation
              myDS.Tables["Customers"].Columns["CustID"], //main columns
              myDS.Tables["Orders"].Columns["OrderID"]); //related columns
          myDS.Relations.Add(dr);
  
          //second relation
          dr = new DataRelation("InventoryOrder",
              myDS.Tables["Inventory"].Columns["CarID"],
              myDS.Tables["Orders"].Columns["OrderID"]);
          //add relations to dataset
          myDS.Relations.Add(dr);
      }
  </code></pre>
  
  <p>Why i got this Null reference Exception? What i miss?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p><img src="https://i.stack.imgur.com/YKRs1.png" alt="enter image description here"></p>