EditProductScreen用于更新现有产品或添加新产品，唯一的区别是更新产品屏幕数据时填充现有产品的初始值以更新它。应用程序正常工作以更新产品，但当我单击按钮添加新产品时，抛出标题中提到的错误。
我使用didChangeDependencies来获取现有产品的ID，并设置其初始值。由于新产品还没有ID，如果条件应用于它，但程序仍然抛出错误。

编辑产品屏幕

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import '../providers/product.dart';
import '../providers/products.dart';

class EditProductScreen extends StatefulWidget {
  static const routeName = '/edit-product-screen';
  //  As form filling will going to take place here we need to show changes on page
  @override
  State<EditProductScreen> createState() => _EditProductScreenState();
}

class _EditProductScreenState extends State<EditProductScreen> {
  final _priceFocusNode = FocusNode();
  final _descriptionFocusNode = FocusNode();
  final _imageUrlController = TextEditingController();
  final _imageUrlFocusNode = FocusNode();
  // FocusNode stick in memory and use much storage (here we are using stateful widget not Provider), so we need to dispose them
  final _form = GlobalKey<FormState>();
  var _editedProduct = Product(
    id: '',
    title: '',
    description: '',
    price: 0,
    imageUrl: '',
  );
  var _isInit = true;
  var _initValues = {
    'title': '',
    'description': '',
    'price': '',
    'imageUrl': '',
  };

  @override
  void dispose() {
    _imageUrlFocusNode.removeListener(
        _updateImageUrl); //  dispose before diaposing _imageUrlFocusNode
    _priceFocusNode.dispose();
    _descriptionFocusNode.dispose();
    _imageUrlController.dispose();
    _imageUrlFocusNode.dispose();
    super.dispose();
  }

  @override
  void didChangeDependencies() {
    if (_isInit) {
      final productId = ModalRoute.of(context)!.settings.arguments as String;
      // print('Siddhant her is your Id $productId');
      if (productId != null) {
        //  if (!productId.isEmpty)
        _editedProduct =
            Provider.of<Products>(context, listen: false).findById(productId);
        _initValues = {
          'title': _editedProduct.title,
          'description': _editedProduct.description,
          'price': _editedProduct.price.toString(),
          // 'imageUrl': _editedProduct.imageUrl,
          'imageUrl': '',
        };
        _imageUrlController.text = _editedProduct.imageUrl;
      }
    }
    _isInit = false;
    super.didChangeDependencies();
  }

  @override
  void initState() {
    _imageUrlFocusNode.addListener(
        _updateImageUrl); //  execute updateImageUrl whenever imageUrlFocusNode chandeg
    super.initState();
  }

  void _updateImageUrl() {
    if (!_imageUrlFocusNode.hasFocus) {
      if ((_imageUrlController.text.isEmpty) ||
          (!_imageUrlController.text.startsWith('http') &&
              !_imageUrlController.text.startsWith('https')) ||
          (!_imageUrlController.text.endsWith('jpeg') &&
              !_imageUrlController.text.endsWith('png') &&
              !_imageUrlController.text.endsWith('jpg'))) {
        return;
      }
      setState(() {});
    }
  }

  void _saveForm() {
    final _isValid = _form.currentState!.validate(); //  trigger all validators
    if (!_isValid) {
      return;
    }
    _form.currentState!.save();
    if (_editedProduct.id != null) {
      Provider.of<Products>(context, listen: false)
          .updateProduct(_editedProduct.id, _editedProduct);
    } else {
      Provider.of<Products>(context, listen: false).addProduct(_editedProduct);
    }
    Navigator.of(context).pop();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Edit Products'),
        actions: [
          // TextButton(
          //   onPressed: () {
          //     Navigator.of(context).pop();
          //   },
          //   child: Text(
          //     'Cancle',
          //     style: TextStyle(
          //       color: Colors.red,
          //       fontSize: 17,
          //     ),
          //   ),
          // ),
          IconButton(
            onPressed: _saveForm,
            icon: Icon(Icons.save),
          ),
        ],
      ),
      body: Padding(
        padding: EdgeInsets.all(16),
        child: Form(
          key: _form,
          child: ListView(
            children: [
              TextFormField(
                initialValue: _initValues['title'],
                decoration: InputDecoration(labelText: 'Title'),
                textInputAction: TextInputAction.next,
                validator: (value) {
                  if (value!.isEmpty) {
                    return 'Please provide a title';
                  }
                  return null;
                },
                onFieldSubmitted: (_) {
                  FocusScope.of(context).requestFocus(_priceFocusNode);
                },
                onSaved: (newValue) {
                  _editedProduct = Product(
                    id: _editedProduct.id,
                    title: newValue.toString(),
                    description: _editedProduct.description,
                    price: _editedProduct.price,
                    imageUrl: _editedProduct.imageUrl,
                    isFavourite: _editedProduct.isFavourite,
                  );
                },
              ),
              TextFormField(
                initialValue: _initValues['price'],
                decoration: InputDecoration(labelText: 'Price'),
                keyboardType: TextInputType.number,
                textInputAction: TextInputAction.next,
                focusNode: _priceFocusNode,
                validator: (value) {
                  if (value!.isEmpty) {
                    return 'Please Enter a Price.';
                  }
                  if (double.tryParse(value) == null) {
                    //  tryParse returns a null when it fails
                    return 'Please Enter a Valid Price.';
                  }
                  if (double.parse(value) < 0) {
                    return 'Price can not be Negative.';
                  }
                  return null;
                },
                onFieldSubmitted: (_) {
                  FocusScope.of(context).requestFocus(_descriptionFocusNode);
                },
                onSaved: (newValue) {
                  _editedProduct = Product(
                    id: _editedProduct.id,
                    title: _editedProduct.title,
                    description: _editedProduct.description,
                    price: double.parse(newValue.toString()),
                    imageUrl: _editedProduct.imageUrl,
                    isFavourite: _editedProduct.isFavourite,
                  );
                },
              ),
              TextFormField(
                initialValue: _initValues['description'],
                decoration: InputDecoration(labelText: 'Description'),
                maxLines: 3,
                keyboardType: TextInputType.multiline,
                focusNode: _descriptionFocusNode,
                validator: (value) {
                  if (value!.isEmpty) {
                    return 'Please Enter a Description.';
                  }
                  if (value.length < 10) {
                    return 'Should be at least 10 characters long.';
                  }
                  return null;
                },
                onSaved: (newValue) {
                  _editedProduct = Product(
                    id: _editedProduct.id,
                    title: _editedProduct.title,
                    description: newValue.toString(),
                    price: _editedProduct.price,
                    imageUrl: _editedProduct.imageUrl,
                    isFavourite: _editedProduct.isFavourite,
                  );
                },
              ),
              Row(
                crossAxisAlignment: CrossAxisAlignment.end,
                children: [
                  Container(
                    height: 100,
                    width: 100,
                    margin: EdgeInsets.only(top: 8, right: 10),
                    decoration: BoxDecoration(
                      border: Border.all(width: 1, color: Colors.black87),
                    ),
                    child: _imageUrlController.text.isEmpty
                        ? Text('Enter a URL')
                        : FittedBox(
                            child: Image.network(
                              _imageUrlController.text,
                              fit: BoxFit.cover,
                            ),
                          ),
                  ),
                  Expanded(
                    child: TextFormField(
                      decoration: InputDecoration(labelText: 'Image URL'),
                      keyboardType: TextInputType.url,
                      textInputAction: TextInputAction.done,
                      controller:
                          _imageUrlController, //  TextFormField already has a controller but here in case of imageUrl we need our own controller to preview the image
                      focusNode: _imageUrlFocusNode,
                      validator: (value) {
                        if (value!.isEmpty) {
                          return 'Please enter an URL';
                        }
                        if (!value.startsWith('http') &&
                            !value.startsWith('https')) {
                          return 'Enter a valid URL';
                        }
                        if (!value.endsWith('jpeg') &&
                            !value.endsWith('png') &&
                            !value.endsWith('jpg')) {
                          return 'Enter a valid Image URL';
                        }
                        return null;
                      },
                      onSaved: (newValue) {
                        _editedProduct = Product(
                          id: _editedProduct.id,
                          title: _editedProduct.title,
                          description: _editedProduct.description,
                          price: _editedProduct.price,
                          imageUrl: newValue.toString(),
                          isFavourite: _editedProduct.isFavourite,
                        );
                      },
                      // onFieldSubmitted: (_) {
                      //   _saveForm();
                      // },
                    ),
                  ),
                ],
              ),
            ],
          ),
        ),
      ),
    );
  }
}

用户产品项目

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import '../screens/edit_product_screen.dart';
import '../providers/products.dart';

class UserProductItem extends StatelessWidget {
  final String id;
  final String title;
  final String imageUrl;

  UserProductItem(this.id, this.title, this.imageUrl);

  @override
  Widget build(BuildContext context) {
    return ListTile(
      title: Text(title),
      leading: CircleAvatar(
        backgroundImage: NetworkImage(imageUrl),
      ),
      trailing: Container(
        width: 100,
        child: Row(children: [
          IconButton(
            onPressed: () {
              Navigator.of(context)
                  .pushNamed(EditProductScreen.routeName, arguments: id);
              // print(id);
            },
            icon: Icon(Icons.edit),
            color: Theme.of(context).primaryColor,
          ),
          IconButton(
            onPressed: () {
              Provider.of<Products>(context, listen: false).deleteProduct(id);
            },
            icon: Icon(Icons.delete),
            color: Theme.of(context).errorColor,
          ),
        ]),
      ),
    );
  }
}