Handle cancellation of the date/time picker in the Gallery full screen dialog demo (#12248)

examples/flutter_gallery/lib/demo/material/full_screen_dialog_demo.dart

@@ -51,6 +51,7 @@ class DateTimeItem extends StatelessWidget {
                     lastDate: date.add(const Duration(days: 30))
                   )
                   .then<Null>((DateTime value) {
+                    if (value != null)
                       onChanged(new DateTime(value.year, value.month, value.day, time.hour, time.minute));
                   });
                 },
@@ -77,6 +78,7 @@ class DateTimeItem extends StatelessWidget {
                   initialTime: time
                 )
                 .then<Null>((TimeOfDay value) {
+                  if (value != null)
                     onChanged(new DateTime(date.year, date.month, date.day, value.hour, value.minute));
                 });
               },