Flutter 有没有办法帮助我修复类型 null 不是 Map 类型的子类型

我已经做了几个月的Flutter 翼了，我知道如何解决无效的安全问题，但这个问题对我来说真的很复杂，我希望得到帮助.

我的问题是:

每当我try 登录时，我都会收到这样的错误:‘Null’类型不是类型映射、字符串、动态类型的子类型.我尽了最大努力找出哪里出了问题，但我无法， 任何帮助都是拯救生命...

import 'package:cloud_firestore/cloud_firestore.dart';

class User {
 final String email;
 final String uid;
 final String photoUrl;
 final String username;
 final String bio;
 final List followers;
 final List following;

const User({
  required this.email,
  required this.uid,
  required this.photoUrl,
  required this.username,
  required this.bio,
  required this.followers,
  required this.following,
 });

然后我继续在这里

Map<String, dynamic> toJson() => {
    "username": username,
    "uid": uid,
    "email": email,
    "photoUrl": photoUrl,
    "bio": bio,
    "followers": followers,
    "following": following,
  };

   static User fromSnap(DocumentSnapshot snap) {
   var snapshot = snap.data() as Map<String, dynamic>;

  return User(
     username: snapshot['username'],
     uid: snapshot['uid'],
     email: snapshot['email'],
     photoUrl: snapshot['photoUrl'],
     bio: snapshot['bio'],
     followers: snapshot['followers'],
     following: snapshot['following'],
      );
     }
    }

现在我在这一行得到了错误:空类型不是映射&lt；字符串，动态&>；的子类型

var snapshot = snap.data() as Map<String, dynamic>;

这都是我的问题， 提前谢谢你.