Flutter Firebase失败断言:第360行pos 10：'data != null‘

import 'package:flutter/material.dart';
import 'package:firebase_database/firebase_database.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Firebase Example')),
        body: FirebaseDataWidget(),
      ),
    );
  }
}

class FirebaseDataWidget extends StatefulWidget {
  @override
  _FirebaseDataWidgetState createState() => _FirebaseDataWidgetState();
}

class _FirebaseDataWidgetState extends State<FirebaseDataWidget> {
  final DatabaseReference _databaseReference = FirebaseDatabase.instance.reference().child('data');
  String _data = '';

  @override
  void initState() {
    super.initState();
    _databaseReference.once().then((DataSnapshot snapshot) {
      if (snapshot.value != null) {
        setState(() {
          _data = snapshot.value.toString();
        });
      } else {
        setState(() {
          _data = 'No data available';
        });
      }
    }).catchError((error) {
      setState(() {
        _data = 'Failed to load data: $error';
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text(_data),
    );
  }
}