Flutter Substitute for accentColor

I trying to make a light theme and a dark theme for my app, I making an course of flutter and the code it's a little out of date and some commands doesn't work in the actual version of Flutter, in special the Themedata.accentColor or just accentColor who is deprecated in the last Flutter version.

I tried to search for a solution and I found two of them, one in the Flutter docs and other on another question of StackOverflow. I tried both of solutions but I still getting the same error

Error message:

Exception has occurred.
_AssertionError ('package:flutter/src/material/theme_data.dart': Failed assertion: line 412 pos 12: 'colorScheme?.brightness == null || brightness == null || colorScheme!.brightness == brightness': is not true.)

Code:

colorScheme: ColorScheme.fromSwatch().copyWith(
  secondary: Colors.white,
),

My code is:

import 'package:flutter/material.dart';

const brightness = Brightness.dark;
Color primaryColor = const Color(0xFF00C569);
Color lightColor = const Color(0xFFFFFFFF);
Color backgroundColor = const Color(0xFFF5F5F5);
Color dangerColor = const Color(0xFFFF0000);

ThemeData darkTheme() {
  return ThemeData(
    brightness: brightness,
    //iconTheme: const IconThemeData(color: Colors.black),
    //textTheme: const TextTheme(
      /*
      bodyText2: TextStyle(color: Colors.red),
      headline1: TextStyle(fontSize: 78),
      headline2: TextStyle(
        color: Colors.black,
        fontSize: 30,
      ),
      button: TextStyle(color: Colors.green),
    ),
    */
    colorScheme: ColorScheme.fromSwatch().copyWith(
      secondary: Colors.white,
    ),
    primaryColor: primaryColor,
  );
}

Solution

I actually discovered a temporary solution for this. The problem was a possible null value, so the code don't work. The solution basically put a ? after the ColorScheme. For example:

colorScheme: ColorScheme?.fromSwatch().copyWith(
      secondary: Colors.white,
    ),

if still doesn't work, try to add the brightness in the color scheme, for example:

colorScheme: ColorScheme?.fromSwatch().copyWith(
      brightness: Brightness.light,
      secondary: Colors.white,
    ),