Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'

Question

I'm trying to do an env setup to flutter the project with null safety. but this error popped.

this worked well with the project without null safety.

source: https://stacksecrets.com/flutter/environment-configuration-in-flutter-app#:~:text=In%20order%20to%20load%20proper,which%20can%20set%20configuration%20dynamically.&text=Since%20the%20Environment%20class%20has,through%20out%20the%20application%20lifecycle.

class Environment {
  factory Environment() {
    return _singleton;
  }

  Environment._internal();
// here
// Non-nullable instance field 'config' must be initialized.
// Try adding an initializer expression, or add a field initializer in this constructor,or mark it 'late'

  static final Environment _singleton = Environment._internal();

  static const String DEV = 'DEV';
  static const String STAGING = 'STAGING';
  static const String PROD = 'PROD';

  BaseConfig config;

  initConfig(String environment) {
    config = _getConfig(environment);
  }

  BaseConfig _getConfig(String environment) {
    switch (environment) {
      case Environment.PROD:
        return ProdConfig();
      case Environment.STAGING:
        return StagingConfig();
      default:
        return DevConfig();
    }
  }
}