flutter The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(String?)?' in flutter

Question

The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(String?)?'.dartargument_type_not_assignable String newValue. hi, i have the above error when trying to implement a dropdown list menu in flutter

bellow is my code. please i am new to flutter

class _CreateAccountState extends State<CreateAccount> {

  String dropdownvalue = 'Apple';
  var items =  ['Apple','Banana','Grapes','Orange','watermelon','Pineapple'];

    @override
  Widget build(BuildContext context) {
    double h = MediaQuery.of(context).size.height;
    double w = MediaQuery.of(context).size.width;
    return Scaffold( ....


 child: DropdownButton(
                value: dropdownvalue,
                  icon: Icon(Icons.keyboard_arrow_down),
                  items:items.map((String items) {
                       return DropdownMenuItem(
                           value: items,
                           child: Text(items)
                       );
                          }
                          ).toList(),
                        onChanged: (String newValue){
                          setState(() {
                            dropdownvalue = newValue;
                          });
                        },
                      ),

thank you all.

Answer 1

Errors like this are null-safety related, you can learn more about null-safety here.

If you check out the DropdownButton Class in the official docs you can see an example in which the onChanged property is used:

 onChanged: (String? newValue) {
        setState(() {
          dropdownValue = newValue!;
        });
      },

If you checkout the onChanged property implementation:

final ValueChanged<T?>? onChanged;

The T? means it requires a nullable type, like String? instead of a non-nullable type, like String.

Answer 2

The argument type of onChanged is void Function(String?). So you can't assign a function of argument type void Function(String)

So please change the code as following:

(change the String type to String? in "onChanged" argument

class _CreateAccountState extends State<CreateAccount> {

 String dropdownvalue = 'Apple';
 var items =  ['Apple','Banana','Grapes','Orange','watermelon','Pineapple'];

   @override
 Widget build(BuildContext context) {
   double h = MediaQuery.of(context).size.height;
   double w = MediaQuery.of(context).size.width;
   return Scaffold( ....


child: DropdownButton(
               value: dropdownvalue,
                 icon: Icon(Icons.keyboard_arrow_down),
                 items:items.map((String items) {
                      return DropdownMenuItem(
                          value: items,
                          child: Text(items)
                      );
                         }
                         ).toList(),
                       onChanged: (String? newValue){
                         setState(() {
                           dropdownvalue = newValue;
                         });
                       },
                     ),