我正在Flutter上工作TextFormField。我想在下面显示一条错误消息，TextFormField但是当我调试时出现此错误
方法“验证”在null上被调用。接收方：null尝试调用：validate（）
class HomePage extends StatefulWidget {
@override
_HomePageState createState() => _HomePageState();}class _HomePageState extends State<HomePage> {
TextEditingController _titleController;
TextEditingController _descriptionController;
final _formKey = GlobalKey<FormState>();
@override
void initState() {
super.initState();
_titleController = new TextEditingController(text: widget.products.adTitle);
_descriptionController = new TextEditingController(text: widget.products.description); @override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Products")
),
body: Container(
margin: EdgeInsets.all(15.0),
alignment:Alignment.center,
key: _formKey,
child: Column(
children: <Widget>[
TextFormField(
controller: _titleController,
decoration: InputDecoration(labelText: 'Title'),
validator: (text) {
if (text == null || text.isEmpty) {
return 'Text is empty';
}
return null;