颤振JSON解码未处理的异常：类型'Null'不是'String'类型的子类型

Question

我关注video tutorial 制作了一个显示不同鸡尾酒以及如何制作它们的应用程序，但最后我遇到了一个我不知道如何解决的错误。我查找了错误，但我无法找出导致它的原因。 这是我得到的错误；

> Unhandled Exception: type 'Null' is not a subtype of type 'String'

我在 searchpage.dart 上收到此错误，代码如下：

import 'package:flutter/material.dart';
import 'package:google_fonts/google_fonts.dart';
import 'package:http/http.dart';
import 'package:mimir_cocktails/constants.dart';
import 'package:mimir_cocktails/pages/cocktail_page.dart';
import 'package:mimir_cocktails/services/cocktail_manager.dart';
import 'package:mimir_cocktails/services/ingredients.dart';

class SearchPage extends StatefulWidget {
  // SearchPage({Key? key}) : super(key: key);

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

class _SearchPageState extends State<SearchPage> {
  String cocktailName = 'margarita';
  var _deviceWidth;
  var _deviceHeight;

  @override
  Widget build(BuildContext context) {
    _deviceWidth = MediaQuery.of(context).size.width;
    _deviceHeight = MediaQuery.of(context).size.height;
    return Scaffold(
      body: SafeArea(
        child: Center(
          child: Padding(
            padding: EdgeInsets.all(12.0),
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              mainAxisSize: MainAxisSize.max,
              crossAxisAlignment: CrossAxisAlignment.center,
              children: [
                // The Logo
                Container(
                  decoration: BoxDecoration(
                    borderRadius: BorderRadius.circular(100),
                  ),
                  height: _deviceHeight * 0.25,
                  width: _deviceWidth * 0.5,
                  child: Image(image: AssetImage('assets/cocktails_logo.png')),
                ),

                // Title
                Text(
                  'Mimir Cocktails',
                  style: GoogleFonts.raleway(fontSize: 35),
                ),
                SizedBox(height: 32),
                //  Textfield
                TextField(
                  onChanged: (value) {
                    value = cocktailName;
                  },
                  decoration: InputDecoration(
                    contentPadding: EdgeInsets.all(15),
                    hintText: 'Search for a Cocktail',
                    border: kBorderStyle,
                    focusedBorder: kFocusedBorderStyle,
                  ),
                ),
                SizedBox(height: 20),
                // SearchBTN
                ElevatedButton(
                  onPressed: () async {
                    // ignore: unnecessary_null_comparison
                    if (cocktailName == null) return;
                    cocktailName.toLowerCase().replaceAll(' ', '_');

                    CocktailManager cm = CocktailManager();

                    var network = await get(Uri.parse(kMainURL + cocktailName));
                    var jsonData = jsonDecode(network.body);

                    cm.name = jsonData['drinks'][0]['strDrink'];
                    cm.category = jsonData['drinks'][0]['strCategory'];
                    cm.alcoholic = jsonData['drinks'][0]['strAlcoholic'];
                    cm.glassType = jsonData['drinks'][0]['strGlass'];
                    cm.imgURL = jsonData['drinks'][0]['strDrinkThumb'];
                    cm.instructions = jsonData['drinks'][0]['strInstructions'];

                    String strIngredientName, strIngredientMesure;
                    List<Ingredient> ingredientList = [];

                    for (var ings = 1; ings < 16; ings++) {
                      strIngredientName = 'strIngredientName' + ings.toString();
                      strIngredientMesure =
                          'strIngredientMesure' + ings.toString();

                      ingredientList.add(
                        Ingredient(
                          name: jsonData['drinks'][0][strIngredientName],
                          mesure: jsonData['drinks'][0][strIngredientMesure],
                        ),
                      );
                      ingredientList.removeWhere((element) =>
                          // ignore: unnecessary_null_comparison
                          element.name == null && element.mesure == null);
                      ingredientList.forEach((element) {
                        // ignore: unnecessary_null_comparison
                        if (element.mesure == null) {
                          element.mesure = 'as you wish';
                        }
                      });
                    }

                    cm.ingredients = ingredientList;

                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) {
                        return CocktailPage(
                            name: cm.name,
                            category: cm.category,
                            alcoholic: cm.alcoholic,
                            glassType: cm.glassType,
                            imgURL: cm.imgURL,
                            instructions: cm.instructions,
                            ingredients: cm.ingredients);
                      }),
                    );
                  },
                  child: Text(
                    'Search',
                    style: GoogleFonts.ubuntu(fontSize: 25),
                  ),
                  style: ElevatedButton.styleFrom(
                    primary: kAccentColor,
                    shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(25)),
                    minimumSize: Size(_deviceWidth * 0.65, 48),
                  ),
                ),
                SizedBox(height: 15),
                // RandomBTN
                ElevatedButton(
                  onPressed: () async {
                    CocktailManager cm = CocktailManager();

                    var network = await get(Uri.parse(kMainURL + cocktailName));
                    var jsonData = jsonDecode(network.body);
                    cm.name = jsonData['drinks'][0]['strDrink'];
                    cm.category = jsonData['drinks'][0]['strCategory'];
                    cm.alcoholic = jsonData['drinks'][0]['strAlcoholic'];
                    cm.glassType = jsonData['drinks'][0]['strGlass'];
                    cm.imgURL = jsonData['drinks'][0]['strDrinkThumb'];
                    cm.instructions = jsonData['drinks'][0]['strInstructions'];

                    // ignore: unused_local_variable
                    String? strIngredientName, strIngredientMesure;
                    List<Ingredient> ingredientList = [];

                    for (var ings = 1; ings < 16; ings++) {
                      strIngredientName = 'strIngredientName' + ings.toString();
                      strIngredientMesure =
                          'strIngredientMesure' + ings.toString();

                      ingredientList.add(
                        Ingredient(
                          name: jsonData['drinks'][0][strIngredientName],
                          mesure: jsonData['drinks'][0][strIngredientMesure],
                        ),
                      );
                      ingredientList.removeWhere((element) =>
                          // ignore: unnecessary_null_comparison
                          element.name == null && element.mesure == null);
                      ingredientList.forEach((element) {
                        // ignore: unnecessary_null_comparison
                        if (element.mesure == null) {
                          element.mesure = 'as you wish';
                        }
                      });
                    }

                    cm.ingredients = ingredientList;

                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) {
                        return CocktailPage(
                            name: cm.name,
                            category: cm.category,
                            alcoholic: cm.alcoholic,
                            glassType: cm.glassType,
                            imgURL: cm.imgURL,
                            instructions: cm.instructions,
                            ingredients: cm.ingredients);
                      }),
                    );
                  },
                  child: Text(
                    'Feeling Locky',
                    style: GoogleFonts.ubuntu(fontSize: 25),
                  ),
                  style: ElevatedButton.styleFrom(
                    primary: kAccentColor,
                    shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(25)),
                    minimumSize: Size(_deviceWidth * 0.65, 48),
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

这就是我得到错误的地方

ingredientList.add(
    Ingredient(
      name: jsonData['drinks'][0][strIngredientName],
      mesure: jsonData['drinks'][0][strIngredientMesure],
    ),

这是我的 coctail_manager.dart

import 'package:mimir_cocktails/services/ingredients.dart';

class CocktailManager {
  String? name;
  String? category;
  String? alcoholic;
  String? glassType;
  String? imgURL;
  String? instructions;
  List<Ingredient>? ingredients;

  CocktailManager({
    this.name,
    this.category,
    this.alcoholic,
    this.glassType,
    this.imgURL,
    this.instructions,
    this.ingredients,
  });
}

请你看看，让我知道问题是什么 如果你想检查整个应用程序，这是Github repository。

Answer 1

我确定您从 json 接收到 null 值。 所以add this null safety operator on each statement 你在哪里分配值，例如

cm.name = jsonData['drinks'][0]['strDrink'] ?? "";