重新打开对话框时，Flutter gif不会再次播放

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter_gifimage/flutter_gifimage.dart';

class MyDialog extends StatefulWidget {
  @override
  _MyDialogState createState() => _MyDialogState();
}

class _MyDialogState extends State<MyDialog> with SingleTickerProviderStateMixin {
  GifController _controller;

  @override
  void initState() {
    super.initState();
    _controller = GifController(vsync: this);
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  void _resetAnimation() {
    _controller.reset();
    _controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: Text('My Dialog'),
      content: Column(
        children: [
          GifImage(
            controller: _controller,
            image: AssetImage('assets/my_animation.gif'),
          ),
          RaisedButton(
            child: Text('Replay Animation'),
            onPressed: _resetAnimation,
          ),
        ],
      ),
      actions: [
        FlatButton(
          child: Text('Close'),
          onPressed: () {
            Navigator.of(context).pop();
          },
        ),
      ],
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: Scaffold(
      appBar: AppBar(
        title: Text('Flutter GIF Dialog'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Open Dialog'),
          onPressed: () {
            showDialog(
              context: context,
              builder: (BuildContext context) {
                return MyDialog();
              },
            );
          },
        ),
      ),
    ),
  ));
}