[已解決][Flutter] This requires the ‘non-nullable’ language feature to be enabled. Try updating your pubspec.yaml to set the minimum SDK constraint to 2.12.0 or higher, and running ‘pub get’.

這是由於 dart 版本升級之後的程式碼，有部分需要啟用所謂的『空安全』（Null-Safety）才能正常編譯所導致的。

你應該會看見下列訊息：

This requires the 'non-nullable' language feature to be enabled. Try updating your pubspec.yaml to set the minimum SDK constraint to 2.12.0 or higher, and running 'pub get'.

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解決方法：啟用 Null-Safety

1. 確認你的 Dart 版本（以我的例子，需要至少為 2.12）。

dart --version

如版本過低，需要升級至所需的最低版本。

2. 使用以下指令確認你項目中的函式庫、套件是否需要升級到 Null-Safety

dart pub upgrade --null-safety
dart pub get
dart pub outdated --mode=null-safety

3. 遷移（若有報錯，則代表有函式庫、套件不支援 Null-Safety）

dart pub outdated --mode=null-safety

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References

• https://stackoverflow.com/questions/64621051/how-to-enable-null-safety-in-flutter
• https://github.com/flutter/flutter/issues/62414
• https://dart.dev/null-safety/migration-guide

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Read More

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• [已解決] Error: This requires the null safety language feature, which is experimental. You can enable the experiment using the ‘–enable-experiment=non-nullable’ command line option.