Mac OS Big sur Flutter install error, “Bad state: Future already completed”

It looks like the issue is related to antivirus software.

I tried installing from zip but continued running into the exact same issue with any Flutter commands. I had to temporarily disable Avast Security and everything resolved itself.

Link to issue on GitHub.

Update
Disabling the "Scan Secure Connection" option on the Web Shield lets me keep Avast running without any issue with Flutter.

I use Avast Security and had to switch off both File Shield and Web Shield in order to "flutter upgrade" and "flutter pub get" on my project.

Switch off these Core Shields if using Avast Security

I imagine will need to do the same to add new dependencies and possibly while building for web which is definitely a pain.

I had same issue after upgrade to macos "Big Sur". Also, I have Kaspersky Internet Security. When KIS is disabled I get same issue. But after closing the KIS, command

$ flutter doctor

works.