title: DataGridView sorting with nulls in DateTime column
body: <p>I've got a DataGridView control in a Windows forms application. There are four columns with string data and three with DateTime data. I'm adding the rows programmatically using the Rows.Add() method. All of the columns have SortMode set to Automatic. Clicking the column headers to sort just works, except for the one DateTime column that has some nulls. When the user clicks that column's header, it throws an ArgumentException: Object must be of type DateTime.</p>
  
  <p>I know the hard way to get around this: setting all of the SortModes to NotSortable, handling the ColumnHeaderMouseClick event and sorting the whole thing manually. I'm looking for the easy way. </p>
  
  <p>Is there a property or something I can set, or some other relatively simple way to allow this column to sort with nulls in it?</p>