macOS Big Sur: Flutter doctor error – Bad state: Future already completed

If you are running into issues getting Flutter setup for development on macOS Big Sur and you are getting the following error when running the ‘flutter doctor’ command and you have Avast Antivirus running, there is a good chance Avast is your problem.

Error Message:

❯ flutter doctor -v
[✓] Flutter (Channel stable, 1.22.4, on macOS 11.0.1 20B29 darwin-x64, locale en-US)
    • Flutter version 1.22.4 at /Users/<user>/dev/env/flutter
    • Framework revision 1aafb3a8b9 (4 days ago), 2020-11-13 09:59:28 -0800
    • Engine revision 2c956a31c0
    • Dart version 2.10.4

⣽Unhandled exception:
Bad state: Future already completed
#0      _AsyncCompleter.complete (dart:async/future_impl.dart:43:31)
#1      _NativeSocket.startConnect.<anonymous closure>.connectNext.<anonymous closure> (dart:io-patch/socket_patch.dart:682:23)
#2      _NativeSocket.issueWriteEvent.issue (dart:io-patch/socket_patch.dart:1102:14)
#3      _NativeSocket.issueWriteEvent (dart:io-patch/socket_patch.dart:1109:12)
#4      _NativeSocket.multiplex (dart:io-patch/socket_patch.dart:1130:11)
#5      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

Once I uninstalled Avast, the ‘flutter doctor’ command works without issue.