EditProductScreen is used for both to update the existing product or to add new product, just difference is while updating product screen data is filled with initial values of existing product to update it. App is working properly to update the product but, throws error mentioned in title when I click button to add new product.

I used didChangeDependencies for getting id of existing product and also to set it’s initial values.
As new product doesn’t have id yet, if condition is applied in it, but still programm throws the error.

EditProductScreen

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();
                      // },
                    ),
                  ),
                ],
              ),
            ],
          ),
        ),
      ),
    );
  }
}

UserProductItem

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,
          ),
        ]),
      ),
    );
  }
}