NoSuchMethodError：方法“ existsSync”在null上被调用

Question

我是新手。我正在创建一个将用户数据存储在Firestore中的应用程序，用户也可以更新它。该应用程序可以将数据存储在Firestore中，并可以从中检索数据，但在更新过程中会引发错误

I/flutter (15042): NoSuchMethodError: The method 'existsSync' was called on null.
I/flutter (15042): Receiver: null
I/flutter (15042): Tried calling: existsSync()  

我下面使用的代码未显示任何编译错误，但仅在更新数据时抛出错误

这是用户添加数据并显示检索数据以进行编辑的表格。

class AddProductsForm extends StatefulWidget {
AddProductsForm(
  {this.submitFn,
  this.initTitle,
  this.initCollection,
  this.initPrice,
  this.initQuantity,
  this.initLiters,
  this.initImage});

final void Function(
   String productTitle,
   String price,
   String quantity,
   String category,
   String liters,
   File image,
   BuildContext ctx,
   bool update,
 ) submitFn;

  final initTitle;
  final initCollection;
  final initPrice;
  final initQuantity;
  final initLiters;
  final initImage;

@override
_AddProductsFormState createState() => _AddProductsFormState();
}

class _AddProductsFormState extends State<AddProductsForm> {
final _formKey = GlobalKey<FormState>();

String _title = '';
String _price = '';
String _quantity = '';

var _category;
String _liters = '';

File _image;


var _update = false;

void _updateValidator() {
FocusScope.of(context).unfocus();

final isValid = _formKey.currentState.validate();

if(isValid){
  _formKey.currentState
      .save();
   }
 }

void _updateSubmit() {
  widget.submitFn(
    _title,
    _price,
    _quantity,
    _category,
    _liters,
    _image,
    context,
    _update,
   );
  }

void _trySubmit() {
  FocusScope.of(context).unfocus();

    final isValid = _formKey.currentState.validate();

if (isValid) {
  _formKey.currentState
      .save(); // this will go and trigger all the onSaved in the TextFormField
  if (_image == null) {
    Scaffold.of(context).showSnackBar(
      SnackBar(
        content: Text('Please pick an Image'),
        backgroundColor: Theme.of(context).errorColor,
      ),
    );
    return;
  }
  widget.submitFn(
    _title,
    _price,
    _quantity,
    _category,
    _liters,
    _image,

    context,
    _update,

   );
 }
}

@override
Widget build(BuildContext context) {


return Form(
  key: _formKey,
  child: Center(
    child: Card(
      elevation: 15,
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(20.0),
      ),
      child: Padding(
        padding: EdgeInsets.all(20),
        child: SingleChildScrollView(
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: <Widget>[
              Flexible(
                fit: FlexFit.loose,
                child: TextFormField(
                  initialValue:
                      widget.initTitle == 'App Bar' ? null : widget.initTitle,
                  decoration: InputDecoration(
                      labelText: 'Enter Product Name',
                      contentPadding: EdgeInsets.all(5)),
                  validator: (value) {
                    if (value.isEmpty) {
                      return 'Enter a the Product Title';
                    }
                    return null;
                  },
                  keyboardType: TextInputType.text,

                  onSaved: (value) {
                    _title = value;
                  }, //this (value) in the (arguments) is the value given by the user
                ),
              ),
              SizedBox(
                height: 30,
              ),
   Container(
                padding: EdgeInsets.only(top: 25),
                alignment: Alignment.bottomRight,
                child: FlatButton.icon(
                  onPressed: () {

                    _updateValidator();
                    print(_quantity);
                    print(widget.initQuantity);

                    if(widget.initTitle != _title )
                    {
                      setState(() {
                        _update = true;
                      });
                      _cmonSubmit();
                      print(_update);
                      print(_quantity);
                      print(widget.initQuantity);
                      setState(() {
                        _update = false;
                      });
                    }else{
                    _trySubmit();
                    }
                    Navigator.of(context).pop();
                  },
                  icon: Icon(Icons.save),
                  label: Text('Save'),
                  color: Colors.grey,
                ),
              ),

下面的代码更新Firestore中的文档字段

在控制台上没有打印打印（更新）。 （更新是一个布尔值）

void _submitProductForm(
String productTitle,
String price,
String quantity,
String category,
var liters,
File image,
BuildContext ctx,
bool update,
) async {

print(update);

try{

  final ref = FirebaseStorage.instance
        .ref()
        .child('product_Image')
        .child(productTitle + '.jpg');

    await ref.putFile(image).onComplete;

    final url = await ref.getDownloadURL();

    if(update == true){
       Firestore.instance.collection(category).document(productTitle).updateData({
        'title': productTitle,
        'price': price,
        'quantity': quantity,
        'category': category,
        'image':url,
        'liters': liters,
        'update': 'true',
      }).catchError((error){
        print(error);
      }).then((value) => print('updated'),);
    }
   }
  }

  @override
  Widget build(BuildContext context) {

return Scaffold(
  body: AddProductsForm(
        submitFn: _submitProductForm,
         ),
       ],
     ), 
   );
 }
}