As the error said, flutter_cli need null safety support. To enable null safety you should update your minimum sdk to 2.12.0. Open your pubspec.yaml then set the sdk constraints to:

environment:
  sdk: '>=2.12.0 <3.0.0'

From official documentation:

From Dart 2.12 to 2.19, you need to enable null safety. You cannot use null safety in SDK versions earlier than Dart 2.12.

To enable sound null safety, set the SDK constraint lower-bound to a language version of 2.12 or later.

Reference: Sound null safety | Dart