title: Setting DataGridView.DefaultCellStyle.NullValue to null at designtime raises error at adding rows runtime
body: <p>In Visual Studio 2008</p>
  
  <ul>
  <li>add a new DataGridView to a form</li>
  <li>Edit Columns</li>
  <li>Add a a new DataGridViewImageColumn</li>
  <li>Open the CellStyle Builder of this column (DefaultCellStyle property)</li>
  <li>Change the NullValue from System.Drawing.Bitmap to null</li>
  <li>Try to add a new Row to the DataGridView at runtime
  (dataGridView1.Rows.Add();)</li>
  <li>You get this error: System.FormatException: Formatted
  value of the cell has a wrong type.</li>
  </ul>
  
  <p>If you change back the NullValue to System.Drawing.Bitmap (as it was) you still get the same error at adding a row.</p>
  
  <p>If you set the NullValue at runtime instead of designtime you don't get anny error. 
  (dataGridView1.Columns[0].DefaultCellStyle.NullValue = null;)</p>
  
  <p>Could you tell me why is that?</p>