anchor.diff:  ///                     child: Text(value),
   ///                   );
   ///                 }).toList(),
  -///                 validator: (String value) {
  +///                 validator: (String? value) {
   ///                   if (value == null) {
   ///                     return 'Must make a selection.';
   ///                   }
  
positive.code: ///               ElevatedButton(
  ///                 onPressed: () {
  ///                   FocusScope.of(context).requestFocus(new FocusNode());
  ///                   if (!_formKey.currentState.validate()) {
  ///                     return;
  ///                   }
  ///                   showDialog<void>(
  
negative.code: ///                 iconSize: 24,
  ///                 elevation: 16,
  ///                 style: TextStyle(color: Colors.deepPurple),
  ///                 onChanged: (String? newValue) {
  ///                   setState(() {
  ///                     _dropdownValue = newValue;
  ///                   });
  