title: Can't add more than one row to DataSet
body: <p>Here's the code:</p>
  
  <pre><code>DataSet1.CashRow CashRow = MainDataSet.Cash.NewCashRow();
  CashRow.SetIdNull();
  CashRow.Date = CashItem.Date;
  CashRow.Description = CashItem.Description;
  CashRow.Amount = CashItem.Amount;
  MainDataSet.Cash.Rows.Add(CashRow);
  </code></pre>
  
  <p>It works just fine for the first time. However, if put in a cycle and simply doubled, no more rows are added. I have to close application and start it again.</p>
  
  <p>Update: I have a DataGridView which is connected to the DataSet. Could this cause any problems? This issue is really weird, been trying to fix this for a week now. Maybe there are another ways to add rows?</p>
  
  <p>Update 2: found the cause: "System.Data.ConstraintException: Column 'Id' is constrained to be unique.  Value '' is already present.".</p>