在dart中解析对象（不支持的操作：无法添加到定长列表中）

我有一个用户对象，该用户对象在用户登录/注册时保存到了Cloud Firestore数据库。因此，当他登录时从数据库中检索用户对象，并且一切正常，直到我尝试对列表“ usersProject”执行“ add”操作：

// Add the new project ID to the user's project list
user.userProjectsIDs.add(projectID);

所以我得到了一个例外，Unhandled Exception: Unsupported operation: Cannot add to a fixed-length list我认为问题在于将用户从json转换为对象时，因为当用户注册时，对象会转换为json并存储在数据库中，并且用户将在转换之前使用该对象自动登录。

void createUser(String email, String password, String username, String name, String birthDate) async {
try {
  // Check first if username is taken
  bool usernameIsTaken = await UserProfileCollection()
      .checkIfUsernameIsTaken(username.toLowerCase().trim());
  if (usernameIsTaken) throw FormatException("Username is taken");

  // Create the user in the Authentication first
  final firebaseUser = await _auth.createUserWithEmailAndPassword(
      email: email.trim(), password: password.trim());

  // Encrypting the password
  String hashedPassword = Password.hash(password.trim(), new PBKDF2());

  // Create new list of project for the user
  List<String> userProjects = new List<String>();

  // Create new list of friends for the user
  List<String> friends = new List<String>();

  // Creating user object and assigning the parameters
  User _user = new User(
    userID: firebaseUser.uid,
    userName: username.toLowerCase().trim(),
    email: email.trim(),
    password: hashedPassword,
    name: name,
    birthDate: birthDate.trim(),
    userAvatar: '',
    userProjectsIDs: userProjects,
    friendsIDs: friends,
  );

  // Create a new user in the fire store database
  await UserProfileCollection().createNewUser(_user);
  
  // Assigning the user controller to the 'user' object
    Get.find<UserController>().user = _user;
    Get.back();

} catch (e) {
  print(e.toString());
}}

当用户注销后，他登录并尝试对用户对象进行操作，这带来了一些属性（列表类型）无法使用的问题。此代码创建项目并添加projectID到用户列表中

  Future<void> createNewProject(String projectName, User user) async {

String projectID = Uuid().v1(); // Project ID, UuiD is package that generates random ID

// Add the creator of the project to the members list and assign him as admin
var member = Member(
  memberUID: user.userID,
  isAdmin: true,
);
List<Member> membersList = new List();
membersList.add(member);

// Save his ID in the membersUIDs list
List <String> membersIDs = new List();
membersIDs.add(user.userID);

// Create chat for the new project
var chat = Chat(chatID: projectID);

// Create the project object
var newProject = Project(
  projectID: projectID,
  projectName: projectName,
  image: '',
  joiningLink: '$projectID',
  isJoiningLinkEnabled: true,
  pinnedMessage: '',
  chat: chat,
  members: membersList,
  membersIDs: membersIDs,
);


// Add the new project ID to the user's project list
user.userProjectsIDs.add(projectID);

try {
  // Convert the project object to be a JSON
  var jsonUser = user.toJson();

  // Send the user JSON data to the fire base
  await Firestore.instance
      .collection('userProfile')
      .document(user.userID)
      .setData(jsonUser);

  // Convert the project object to be a JSON
  var jsonProject = newProject.toJson();

  // Send the project JSON data to the fire base
  return await Firestore.instance
      .collection('projects')
      .document(projectID)
      .setData(jsonProject);
} catch (e) {
  print(e);
}}

在这里只有当用户注销然后登录时才会发生例外，但是当他第一次注册时就不会有例外。

 // Add the new project ID to the user's project list
user.userProjectsIDs.add(projectID);

登录功能

void signIn(String email, String password) async {
try {
  // Signing in
  FirebaseUser firebaseUser = await _auth.signInWithEmailAndPassword(email: email.trim(), password: password.trim());

  // Getting user document form firebase
  DocumentSnapshot userDoc = await UserProfileCollection().getUser(firebaseUser.uid);
 

  // Converting the json data to user object and assign the user object to the controller
  Get.find<UserController>().user = User.fromJson(userDoc.data);
  print(Get.find<UserController>().user.userName);

} catch (e) {
  print(e.toString());
}}

我认为，User.fromJson为什么它会使Firestore中的数组不可修改而引起的问题？

用户类别

class User {
  String userID;
  String userName;
  String email;
  String password;
  String name;
  String birthDate;
  String userAvatar;
  List<String> userProjectsIDs;
  List<String> friendsIDs;

  User(
      {this.userID,
      this.userName,
      this.email,
      this.password,
      this.name,
      this.birthDate,
      this.userAvatar,
      this.userProjectsIDs,
      this.friendsIDs});

  User.fromJson(Map<String, dynamic> json) {
    userID = json['userID'];
    userName = json['userName'];
    email = json['email'];
    password = json['password'];
    name = json['name'];
    birthDate = json['birthDate'];
    userAvatar = json['UserAvatar'];
    userProjectsIDs = json['userProjectsIDs'].cast<String>();
    friendsIDs = json['friendsIDs'].cast<String>();
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['userID'] = this.userID;
    data['userName'] = this.userName;
    data['email'] = this.email;
    data['password'] = this.password;
    data['name'] = this.name;
    data['birthDate'] = this.birthDate;
    data['UserAvatar'] = this.userAvatar;
    data['userProjectsIDs'] = this.userProjectsIDs;
    data['friendsIDs'] = this.friendsIDs;
    return data;
  }
}