title: Datagridview column does not allow null
body: <p>I have a datagridview bound to a datasource, column[0] AgencyName is a primary key and cannot be null.
  Clicking the Bindingnavigator ( + ) plus sign to add new row (twice) will result to error "Column (AgencyName) does not allow null". How to handle this null exception?</p>
  
  <p>Overall can you show me how to handle null and  primary key constraint in a datagridview bound to a datasource. Note that I just dragged a datasource into a winform to create this. </p>
  
  <p><img src="https://i.stack.imgur.com/NaVwV.jpg" alt="Form"></p>
  
  <p>I have this code</p>
  
  <pre><code>    private void clientsDataGridView_Validating(object sender, CancelEventArgs e)
      {
  
          DataGridViewRow row = clientsDataGridView.CurrentRow as DataGridViewRow;
  
          if (row.Cells[0].Value == DBNull.Value)
          {
  
             clientsDataGridView.CancelEdit();                
             e.Cancel = true;
  
          }
       }
  </code></pre>
  
  <p>this code wont let me navigate through the other records unless i provide a valid value. I want the new row (with invalid data) to be cancel whenever i move to the other rows</p>