所以我使用的是simple_animations: ^5.0.2包，但我尝试运行的项目是在一个旧版本的简单动画上制作的，所以我对这个包没有太多的了解，但我试图模仿旧版本的功能：

import 'package:flutter/material.dart';
import 'package:simple_animations/simple_animations.dart';

class FadeAnimation extends StatelessWidget {
  final double delay;
  final Widget child;

  FadeAnimation(this.delay, this.child);

  @override
  Widget build(BuildContext context) {
    final tween = MovieTween();

    tween.tween('opacity', Tween(begin: 0.0, end: 1.0), duration: Duration(milliseconds: 500));
    tween.tween('translateY', Tween(begin: -30.0, end: 0.0), duration: Duration(milliseconds: 500), curve: Curves.easeOut);

    return PlayAnimationBuilder(
      delay: Duration(milliseconds: (500 * delay).round()),
      duration: tween.duration,
      tween: tween,
      child: child,
      builder: (context, movie, child) => Opacity(
        opacity: child['opacity'], // The method '[]' can't be unconditionally invoked because the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null check to the target ('!').
        child: Transform.translate(
            offset: Offset(0, child["translateY"]), child: child), // The method '[]' can't be unconditionally invoked because the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null check to the target ('!').
      ),
    );
  }
}

我在第23行和第25行得到了错误，它们基本上都说了同样的话，“方法'[]'不能被无条件调用，因为接收者可以是'null'。尝试使调用有条件（使用'？.'）或向目标（'！我不知道该怎么办，也不知道该怎么办。

旧版本

import 'package:flutter/material.dart';
import 'package:simple_animations/simple_animations.dart';

class FadeAnimation extends StatelessWidget {
  final double delay;
  final Widget child;

  FadeAnimation(this.delay, this.child);

  @override
  Widget build(BuildContext context) {
    final tween = MultiTrackTween([
      Track("opacity").add(Duration(milliseconds: 500), Tween(begin: 0.0, end: 1.0)),
      Track("translateY").add(
        Duration(milliseconds: 500), Tween(begin: -30.0, end: 0.0),
        curve: Curves.easeOut)
    ]);

    return ControlledAnimation(
      delay: Duration(milliseconds: (500 * delay).round()),
      duration: tween.duration,
      tween: tween,
      child: child,
      builderWithChild: (context, child, animation) => Opacity(
        opacity: animation["opacity"],
        child: Transform.translate(
          offset: Offset(0, animation["translateY"]), 
          child: child
        ),
      ),
    );
  }
}