Error: This requires the 'non-nullable' experiment to be enabled

This problem was occured for me after upgrading the Flutter.
I solved it by cleaning and upgrading the project dependencies again.
Run the below commands in the root directory of your project:

flutter clean
flutter packages pub upgrade
flutter pub run build_runner build

Also as others said, please make sure your sdk version in the pubspec.yaml is compatible with your flutter sdk version.

For null safety to work,

environment:
   sdk: ">=2.12.0 <3.0.0"

should be this version at least. then run flutter clean and flutter pub get it will work.

Try

flutter clean

This is worked for me!

Depending on what you're doing, sometimes it's as simple as changing the environment in your pubspec.yaml file to update the lower end, ie change a lower end like

environment:
sdk: ">=2.0.0 <3.0.0"

to

environment:
sdk: ">=2.6.0 <3.0.0"

or

environment:
sdk: ">=2.7.0 <3.0.0"

This has worked for the various things that cause this error (using spread syntax in latest version of flutter and dart will also cause this), but I don't need the older environments.