相关疑难解决方法(0)

这是错误：

有问题的小部件是：FutureBuilder 构建函数绝不能返回 null。要返回导致建筑小部件填充可用空间的空白空间，请返回“Container()”。要返回占用尽可能少空间的空白空间，请返回“Container(width: 0.0, height: 0.0)”。

这是获取 Firebase 实时数据的函数：

Future<void> getCategoriesName() async {
   
    await  FirebaseDatabase.instance.reference().child('Categories').once()
        .then((DataSnapshot dataSnapshot){
      var key  = dataSnapshot.value.keys;
      for(var i in key)
      {
        CategoryItems categoryItems =  new CategoryItems(
            dataSnapshot.value[i]['CategoryName'],
            dataSnapshot.value[i]['Counter']
            

        );
        categoryItemList.add(categoryItems);
      }
      setState(() {
        print(categoryItemList.length);
      });

    });
  }

这是我的 Future Builder 功能：

 Container(
                        margin: EdgeInsets.only(bottom: 50,top: 100),
                        child: FutureBuilder(
                          future: FirebaseDatabase.instance.reference().child('Categories').child(_userPin).once(),
                          // ignore: missing_return
                          builder: (context, snapshot) {
                            if(snapshot.hasData){
                               if (snapshot.data!=null) {
                                return Expanded();
                               }else{
                                 return Loader();
                               }
                          }
                          }
                        ),
                      ),

在 Expanded 内部我有 …

我正在尝试解析 JSON 并将其转换为列表视图。

我正在获取响应正文，它也被转换为列表，但它向未来的构建器发送空值，我收到此错误：

/flutter ( 9715): ??? EXCEPTION CAUGHT BY WIDGETS LIBRARY ????????????????????????????????????????????????????????????
I/flutter ( 9715): The following assertion was thrown building FutureBuilder<List<Event>>(dirty, state:
I/flutter ( 9715): _FutureBuilderState<List<Event>>#6efa6):
I/flutter ( 9715): A build function returned null.
I/flutter ( 9715): The offending widget is: FutureBuilder<List<Event>>
I/flutter ( 9715): Build functions must never return null. To return an empty space that causes the building widget to
I/flutter ( 9715): fill available room, return "new Container()". To return an empty space that …