错误：list <dynamic>不是'string'类型的子类型

Question

这里有我的两个Dart文件，我从firestore中检索了我的视频，但是在这种情况下，我制作了视频数组并尝试对其进行检索，但是如果我使用字符串而不是数组来执行，它会给我一个错误。很好，但是在我的项目中，我需要数组，所以请帮我指点一下。

代码：1个main.dart

import 'dart:io';

import 'chewie_list_item.dart';
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:video_player/video_player.dart';

void main() {
  FlutterError.onError = (FlutterErrorDetails details) {
    FlutterError.dumpErrorToConsole(details);
    if (kReleaseMode) exit(1);
  };

  runApp(MaterialApp(
    home: CourseApp(),
  ));
}

class CourseApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Flutter Demo"),
        ),
        body: StreamBuilder(
          stream: Firestore.instance.collection("courses").snapshots(),
          builder: (context, snapshot) {
            return ListView.builder(
              itemCount: snapshot.data.documents.length,
              itemBuilder: (context, index) {
                DocumentSnapshot courses = snapshot.data.documents[index];
                return (ChewieListItem(
                  videoPlayerController: VideoPlayerController.network(
                      courses['video'] ?? 'default'),
                ));
              },
            );
          },
        ));
  }
}

[code 2: chewie_list_item.dart]

import 'package:chewie/chewie.dart';
import 'package:flutter/material.dart';
import 'package:video_player/video_player.dart';

class ChewieListItem extends StatefulWidget {
  // This will contain the URL/asset path which we want to play
  final VideoPlayerController videoPlayerController;
  final bool looping;

  ChewieListItem({
    @required this.videoPlayerController,
    this.looping,
    Key key,
  }) : super(key: key);

  @override
  _ChewieListItemState createState() => _ChewieListItemState();
}

class _ChewieListItemState extends State<ChewieListItem> {
  ChewieController _chewieController;

  @override
  void initState() {
    super.initState();
    // Wrapper on top of the videoPlayerController
    _chewieController = ChewieController(
      videoPlayerController: widget.videoPlayerController,
      aspectRatio: 16 / 9,
      // Prepare the video to be played and display the first frame
      autoInitialize: true,
      looping: widget.looping,
      // Errors can occur for example when trying to play a video
      // from a non-existent URL
      errorBuilder: (context, errorMessage) {
        return Center(
          child: Text(
            errorMessage,
            style: TextStyle(color: Colors.white),
          ),
        );
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: Chewie(
        controller: _chewieController,
      ),
    );
  }

  @override
  void dispose() {
    super.dispose();
    // IMPORTANT to dispose of all the used resources
    widget.videoPlayerController.dispose();
    _chewieController.dispose();
  }
}

Here I am retrieving my video array from a firebase but I get this error please give me a solution of this code.