Fix NPEs in DatePicker, TimePicker demos (#6509)

examples/flutter_gallery/lib/demo/date_picker_demo.dart

@@ -24,7 +24,7 @@ class _DatePickerDemoState extends State<DatePickerDemo> {
       firstDate: new DateTime(2015, 8),
       lastDate: new DateTime(2101)
     );
-    if (picked != _selectedDate) {
+    if (picked != null && picked != _selectedDate) {
       setState(() {
         _selectedDate = picked;
       });

examples/flutter_gallery/lib/demo/time_picker_demo.dart

@@ -21,7 +21,7 @@ class _TimePickerDemoState extends State<TimePickerDemo> {
       context: context,
       initialTime: _selectedTime
     );
-    if (picked != _selectedTime) {
+    if (picked != null && picked != _selectedTime) {
       setState(() {
         _selectedTime = picked;
       });