■ 参考リンク
- https://gist.github.com/masashi-sutou/5807334b4ac814b15c2c43e92012808a
- https://api.flutter.dev/flutter/material/showDialog.html
- https://www.kikt.top/posts/flutter/dialog/dialog-2/
- https://medium.com/flutter-community/how-to-animate- dialogs-in-flutter-here-is-answer-492ea3a7262f