You need to return some type of alternate value when you have a field that will support NULL values, this way you can avoid this error. You can use a nullable generic or just returned a specified value when the value is DBNull. To do this you will need to do the following:
You could also return an alternate value by selecting the "Return Alternate On Null" and then setting the "Null Replacement" value to #01/01/1800#
FYI: All StrataFrame controls will respect #01/01/1800# as a NULL value so you can clear out the contents of a DateTime field by setting the ClearOnNull property. If you do not want to use 01/01/1800 as the null date, you can set it to whatever you like and then set the NullDateValue property on the DateTimePicker to the date you prefer to use a "null".
Hope this helps.