Flutter Error: Type '_i4.RouteConfig' not found

Question

Getting this error while attempting to run my flutter app: (https://i.sstatic.net/9LKyH.png)

Flutter doctor is healthy:

All dependencies are on flutter pub get --major-versions

Here's my pubspec.yaml:

name: flutter project
description: A new Flutter project.

# The following line prevents the package from being accidentally published to
# pub.dev using `flutter pub publish`. This is preferred for private packages.
publish_to: 'none' # Remove this line if you wish to publish to pub.dev

# The following defines the version and build number for your application.
# A version number is three numbers separated by dots, like 1.2.43
# followed by an optional build number separated by a +.
# Both the version and the builder number may be overridden in flutter
# build by specifying --build-name and --build-number, respectively.
# In Android, build-name is used as versionName while build-number used as versionCode.
# Read more about Android versioning at https://developer.android.com/studio/publish/versioning
# In iOS, build-name is used as CFBundleShortVersionString while build-number used as CFBundleVersion.
# Read more about iOS versioning at
# https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
version: 1.0.0+1

environment:
  sdk: ">=3.0.0 <4.0.0"

# Dependencies specify other packages that your package needs in order to work.
# To automatically upgrade your package dependencies to the latest versions
# consider running `flutter pub upgrade --major-versions`. Alternatively,
# dependencies can be manually updated by changing the version numbers below to
# the latest version available on pub.dev. To see which dependencies have newer
# versions available, run `flutter pub outdated`.
dependencies:
  flutter:
    sdk: flutter
  auto_route: ^7.8.0
  copy_with_extension: ^5.0.3
  easy_sidemenu: ^0.5.0
  equatable: ^2.0.5
  feather_icons: ^1.2.0
  flutter_bloc: ^8.1.3

  #API calling
  http: ^1.1.0
  http_interceptor: ^2.0.0-beta.7
  responsive_grid: ^2.3.0

  #Local storage
  shared_preferences: ^2.2.0

  #Image view
  flutter_svg: ^2.0.7
  cached_network_image: ^3.2.3
  shimmer: ^3.0.0

  rxdart: ^0.27.7

  # Responsive UI :
  responsive_sizer: ^3.1.1
  font_awesome_flutter: ^10.4.0
  dropdown_button2: ^2.1.3
  number_pagination: ^0.0.3+2
  responsive_table: ^0.2.0+2
  analog_clock_picker: ^0.0.6

  #Auto complete textField tags
  textfield_tags: ^2.0.2

  #Charts
  syncfusion_flutter_charts: ^22.2.8

  #Toast messages
  bot_toast: ^4.0.4

  #Pick images for web:
  image_picker_web: ^3.0.0+1

  #Date format
  intl: ^0.18.1

  #Scan QR Codes
  qr_bar_code_scanner_dialog: ^0.0.5


  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^1.0.2
  auto_route_generator: ^7.3.1

dev_dependencies:
  build_runner: ^2.1.8
  copy_with_extension_gen: ^5.0.2
  flutter_test:
    sdk: flutter

  # The "flutter_lints" package below contains a set of recommended lints to
  # encourage good coding practices. The lint set provided by the package is
  # activated in the `analysis_options.yaml` file located at the root of your
  # package. See that file for information about deactivating specific lint
  # rules and activating additional ones.
  flutter_lints: ^2.0.0

# For information on the generic Dart part of this file, see the
# following page: https://dart.dev/tools/pub/pubspec

# The following section is specific to Flutter packages.
flutter:

  # The following line ensures that the Material Icons font is
  # included with your application, so that you can use the icons in
  # the material Icons class.
  uses-material-design: true

  # To add assets to your application, add an assets section, like this:
  assets:
    - assets/images/
    - assets/icons/
  #   - images/a_dot_burr.jpeg
  #   - images/a_dot_ham.jpeg

  # An image asset can refer to one or more resolution-specific "variants", see
  # https://flutter.dev/assets-and-images/#resolution-aware

  # For details regarding adding assets from package dependencies, see
  # https://flutter.dev/assets-and-images/#from-packages

  # To add custom fonts to your application, add a fonts section here,
  # in this "flutter" section. Each entry in this list should have a
  # "family" key with the font family name, and a "fonts" key with a
  # list giving the asset and other descriptors for the font. For
  # example:
  fonts:
    - family: Futura
      fonts:
        - asset: assets/fonts/futura_light_bt.ttf
          weight: 400
        - asset: assets/fonts/futura medium bt.ttf
          weight: 500
        - asset: assets/fonts/Futura Bold font.ttf
          weight: 700
  # fonts:
  #   - family: Schyler
  #     fonts:
  #       - asset: fonts/Schyler-Regular.ttf
  #       - asset: fonts/Schyler-Italic.ttf
  #         style: italic
  #   - family: Trajan Pro
  #     fonts:
  #       - asset: fonts/TrajanPro.ttf
  #       - asset: fonts/TrajanPro_Bold.ttf
  #         weight: 700
  #
  # For details regarding fonts from package dependencies,
  # see https://flutter.dev/custom-fonts/#from-packages

I've tried adding both of these packages: https://pub.dev/packages/router_generator/install https://pub.dev/packages/auto_route_generator/install

This gives an error saying that I can't run with sound null safety, because the dependencies don't support null safety. To solve, tried adding to pubspec.yaml:

analyzer:
  enable-experiment:
  - non-nullable

When re-running, I get this error:

flutter run --no-sound-null-safety
Could not find an option named "no-sound-null-safety".
Run 'flutter -h' (or 'flutter <command> -h') for available flutter commands and options.

The only solution I've found: https://www.devopsschool.com/blog/flutter-cannot-run-with-sound-null-safety-because-the-following-dependencies-dont-support-null-safety/

Is to add

// @dart=2.9

To top of main.dart. Doing so gives me this error:

Here's what happens when I run

flutter pub run build_runner build --delete-conflicting-outputs

Any suggestions?