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

Question

I am trying to add DropdownButton widget to my app.

This is code for my app

import 'package:flutter/material.dart';

void main()
{
  runApp(MaterialApp(
    home: Home(),
  ));
}

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {

  var gadgets = ['Laptop', 'Cell-phone', 'Drone', 'earbuds', 'walkman'];
  var _currentGadgets = 'Gadgets';


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Dropdown Button'),
        centerTitle: true,
      ),

      body: Column(
        children: [
         DropdownButton(
           items: gadgets.map((e){
             return DropdownMenuItem(
                 value: e,
                 child: Text(e),
             );
           }).toList(),
           onChanged: (String userchoice){
             setState(() {
               this._currentGadgets = userchoice;
             });
           },
           value: _currentGadgets,
         ),
        ],
      ),

    );
  }
}

When I try to execute this code this error appears The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(String?)?'. How can I solve this problem? I am new to flutter, I am stucked in this code from past 2 days. Please help.

Solution

The error you get came from null-safety, the type String? means that it could be either a String, or null, but your parameter only accepts a String, and no null.

For this, you can "force" the use of 'non-null' variable by adding a ! at the end of your variable, but be careful when doing this.

onChanged: (String? userchoice){
             setState(() {
               this._currentGadgets = userchoice;
             });
           },

You can learn more about null-safety syntax and principles on the official documentation: https://flutter.dev/docs/null-safety.

https://api.flutter.dev/flutter/material/DropdownButton-class.html.