这在Flutter中是零安全的。
有2个选项：

• 使值nullable

class User{
  final String? fullName;
  final String? email;
  final String? phone;
  final int? id;

  User({
    this.email,
    this.fullName,
    this.phone,
    this.id,
  });

这样，如果快照中没有数据，则user中的变量可以为null

• 使其非空

class User{
  final String fullName;
  final String email;
  final String phone;
  final int id;

  User({
    required this.email,
    required this.fullName,
    required this.phone,
    required this.id,
  });
// since its required, it cant be null. 
// if snapshot value is null, it will error.
// we can assign another default value

User.fromSnapshot(DataSnapshot snapshot){

    id = snapshot.key ?? -1;
    // if phone is null, its set to empty string
    phone = snapshot.value['phone'] ?? ''; 
    email = snapshot.value['email'] ?? '';
    fullName = snapshot.value['fullname'] ?? ''; 
  }

请尝试以下代码：

import 'package:firebase_database/firebase_database.dart';

class User{
  var fullName;
  var email;
  var phone;
  var id;

  User({
    this.email,
    this.fullName,
    this.phone,
    this.id,
  });

  User.fromSnapshot(DataSnapshot snapshot){

    id = snapshot.key;
    phone = snapshot.value['phone']!;
    email = snapshot.value['email']!;
    fullName = snapshot.value['fullname']!;
  }
}

或

import 'package:firebase_database/firebase_database.dart';

class User{
  var fullName;
  var email;
  var phone;
  var id;

  User({
    this.email,
    this.fullName,
    this.phone,
    this.id,
  });

  User.fromSnapshot(DataSnapshot snapshot){

    id = snapshot.key;
    phone = snapshot.value['phone'] ?? 'phone';
    email = snapshot.value['email'] ?? 'email';
    fullName = snapshot.value['fullname'] ?? 'fullname';
  }
}