fixes [NavigationDrawer selected item has wrong icon color [Material3 spec]](https://github.com/flutter/flutter/issues/129572) ### Description This PR fixes a mistake in the `NavigationDrawer` defaults, where generated token value returns a `null`. This issue can be detected when you want to customize the selected icon color for `NavigationDrawerDestination` using a custom color scheme. ### Code sample <details> <summary>expanded to view the code sample</summary> ```dart import 'package:flutter/material.dart'; void main() => runApp(const MyApp()); class MyApp extends StatelessWidget { const MyApp({super.key}); @override Widget build(BuildContext context) { return MaterialApp( debugShowCheckedModeBanner: false, themeMode: ThemeMode.light, theme: ThemeData( colorScheme: ColorScheme.fromSeed(seedColor: Colors.blue).copyWith( onSecondaryContainer: Colors.red, ), useMaterial3: true, ), home: const Example(), ); } } class Example extends StatelessWidget { const Example({super.key}); @override Widget build(BuildContext context) { return Scaffold( appBar: AppBar( title: const Text('NavigationDrawer Sample'), ), drawer: const NavigationDrawer( children: <Widget>[ NavigationDrawerDestination( icon: Icon(Icons.favorite_outline_rounded), label: Text('Favorite'), selectedIcon: Icon(Icons.favorite_rounded), ), NavigationDrawerDestination( icon: Icon(Icons.favorite_outline_rounded), label: Text('Favorite'), ), ], ), ); } } ``` </details> ### Before <img width="1053" alt="Screenshot 2023-06-27 at 13 24 38" src="https://github.com/flutter/flutter/assets/48603081/18c13a73-688f-4586-bb60-bddef45d173f"> ### After <img width="1053" alt="Screenshot 2023-06-27 at 13 24 25" src="https://github.com/flutter/flutter/assets/48603081/8a1427c6-517f-424a-b0bd-24bad7c5fbb0">
Token Defaults Generator
Script that generates component theme data defaults based on token data.
Usage
Run this program from the root of the git repository:
dart dev/tools/gen_defaults/bin/gen_defaults.dart [-v]
This updates generated/used_tokens.csv and the various component theme files.
Templates
There is a template file for every component that needs defaults from
the token database. These templates are implemented as subclasses of
TokenTemplate. This base class provides some utilities and a structure
for adding a new block of generated code to the bottom of a given file.
Templates need to override the generate method to provide the generated
code block as a string.
See lib/fab_template.dart for an example that generates defaults for the
Floating Action Button.
Tokens
Tokens are stored in JSON files in data/, and are sourced from
an internal Google database.
template.dart should provide nearly all useful token resolvers
(e.g. color, shape, etc.). For special cases in which one shouldn't
be defined, use getToken to get the raw token value. The script, through
the various revolvers and getToken, validates tokens, keeps track of
which tokens are used, and generates generated/used_tokens.csv.