如何修复“方法'call'在null上被调用”。

Question

我用于产品创建和更新（重用）的此文件。当我创建产品时，效果很好。但是，当我单击更新时，以前的产品就会显示错误。

I/flutter ( 8544): ══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
I/flutter ( 8544): The following NoSuchMethodError was thrown while handling a gesture:
I/flutter ( 8544): The method 'call' was called on null.
I/flutter ( 8544): Receiver: null
I/flutter ( 8544): Tried calling: call(_LinkedHashMap len:4)
I/flutter ( 8544):
I/flutter ( 8544): When the exception was thrown, this was the stack:

这是我的完整代码：

import 'package:flutter/material.dart';
import '../widgets/helpers/ensure_visible.dart';

class product_Edit extends StatefulWidget {
  Function addProduct;
  Function updateProduct;
  Map<String, dynamic> product;
  int productIndex;
  product_Edit(
      {this.addProduct, this.productIndex, this.product, this.updateProduct});
  @override
  State<StatefulWidget> createState() {
    return _product_EditState();
  }
}

class _product_EditState extends State<product_Edit> {
  final Map<String, dynamic> _formData = {
    'imageUrl': 'assets/food.jpg',
    'title': Null,
    'description': Null,
    'price': Null
  };
  final _titleFocusNode = FocusNode();
  final _descriptionFocusNode = FocusNode();
  final _priceFocusNode = FocusNode();

  final GlobalKey<FormState> _globalFormKey = GlobalKey<FormState>();

  Widget _productNameTextField() {
    return EnsureVisibleWhenFocused(
      focusNode: _titleFocusNode,
      child: TextFormField(
        focusNode: _titleFocusNode,
        decoration: InputDecoration(labelText: 'Product Name'),
        initialValue: widget.product == null ? '' : widget.product['title'],
        validator: (String value) {
          if (value.isEmpty || value.length < 5) {
            return "Title is required and Should be 5+ characters long!";
          }
        },
        onSaved: (String value) {
          setState(() {
            _formData['title'] = value;
          });
        },
      ),
    );
  }

  Widget _productDescriptionTextField() {
    return EnsureVisibleWhenFocused(
      focusNode: _descriptionFocusNode,
      child: TextFormField(
        focusNode: _descriptionFocusNode,
        decoration: InputDecoration(labelText: 'Product Description'),
        maxLines: 4,
        initialValue:
            widget.product == null ? '' : widget.product['description'],
        validator: (String value) {
          if (value.trim().length < 8) {
            return "Description is required and Should be 8+ characters long!";
          }
        },
        onSaved: (String value) {
          _formData['description'] = value;
        },
      ),
    );
  }

  Widget _productPriceTextField() {
    return EnsureVisibleWhenFocused(
      focusNode: _priceFocusNode,
      child: TextFormField(
        focusNode: _priceFocusNode,
        decoration: InputDecoration(labelText: 'Product Price'),
        initialValue:
            widget.product == null ? '' : widget.product['price'].toString(),
        validator: (String value) {
          if (value.isEmpty ||
              !RegExp(r'^(?:[1-9]\d*|0)?(?:\.\d+)?$').hasMatch(value)) {
            return "Price is required and should be number!";
          }
        },
        onSaved: (String value) {
          _formData['price'] = double.parse(value);
        },
        keyboardType: TextInputType.number,
      ),
    );
  }

  void _submitForm() {
    if (!_globalFormKey.currentState.validate()) {
      return;
    }

    _globalFormKey.currentState.save();

    if (widget.product == null) {
      widget.addProduct(_formData);
    } else if (widget.product != null) {
      //widget.updateProduct(widget.productIndex,_formData);
      widget.addProduct(_formData);
    }
    Navigator.pushReplacementNamed(context, '/products');
  }

  Widget _buildPageContent() {
    final double deviceWidth = MediaQuery.of(context).size.width;
    final double targetWidth = deviceWidth > 500 ? 500 : deviceWidth * 0.95;
    final double targetPadding = deviceWidth - targetWidth;

    return Container(
      padding: EdgeInsets.all(10.0),
      child: Form(
        key: _globalFormKey,
        child: ListView(
          padding: EdgeInsets.symmetric(horizontal: targetPadding / 2),
          children: <Widget>[
            _productNameTextField(),
            _productDescriptionTextField(),
            _productPriceTextField(),
            SizedBox(
              height: 6.0,
            ),
            RaisedButton(
                color: Theme.of(context).accentColor,
                child: Text(
                  'Save',
                  style: TextStyle(color: Colors.white),
                ),
                onPressed: _submitForm)
          ],
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    Widget pageContent = _buildPageContent();
    return widget.product == null
        ? pageContent
        : Scaffold(
            appBar: AppBar(
              title: Text('Edit Text'),
            ),
            body: pageContent,
          );
  }
}