The name of the property indicates that it returns the datetime when the control has been closed the last time. It seems that
null
is returned when this has not happend so far. So you must catch that condition in your code:
TimeSpan span = new TimeSpan(0, 0, 0);
if (dropDown != null && dropDown.LastClosedTimeStamp != null)
span = DateTime.Now.Subtract( dropDown.LastClosedTimeStamp );
To check this condition afterwards, you may then use the
TimeSpan.Zero
property.