Firestore Java不支持嵌套数组，调整模型后仍报错求助

Firestore不支持嵌套数组，我将嵌套数组存入单独的Progress类并调整了Assignmentsdata模型，但仍遇到相同错误。尝试通过FieldValue.arrayUnion将Assignmentsdata数组保存到Firestore时失败，相关代码如下：

模型类 #

public class Assignmentsdata implements Serializable {

    String assignmentId;
    String teacherId;
    int game_nameId;
    String sectionId;
    String levelId;
    String startTime;
    String endTime;
    String groupName;

    // 包含嵌套数组的类
    Progress progress;

    int number_of_exercises;
    int exStatus;

    public Assignmentsdata() {

    }
    // getter和setter方法
}

含嵌套数组的类 #

public class Progress {

    public Progress() {
    }

    ArrayList<String> progressList;

    public Progress(ArrayList<String> progressList) {
        this.progressList = progressList;
    }

    public ArrayList<String> getProgressList() {
        return progressList;
    }

    public void setProgressList(ArrayList<String> progressList) {
        this.progressList = progressList;
    }
}

保存数据代码 #

db.collection("users").document(docName).update(
                        "myArchiveList", FieldValue.arrayUnion(assignmentsdata)).
                addOnCompleteListener(new OnCompleteListener<Void>() {
                    @Override
                    public void onComplete(@NonNull Task<Void> task){}

内容的提问来源于stack exchange，提问作者Ifan 767