checkbox not displayed in futureBuilder

I have no idea what is going wrong here. I have two checkbox, one wrap with futureBuilder. But the checkbox wrap with FutureBuilder always can't displayed. If I change to return Text(snapshot.data);, I can see it display as 1 or 0.

But if I change to this, it not displayed anything.

 return Checkbox(
       onChanged: null, 
       value: snapshot.data == "0"
        ? false
        : true,
   );

Full code

 ...item['questions']
                                        .asMap()
                                        .entries
                                        .map((items) {
                                      return TableRow(children: [
                                        Padding(
                                            padding: EdgeInsets.all(10),
                                            child: Text(items.value['name'])),
                                        FutureBuilder(
                                            future: _workOrderBloc.selectAnswer(
                                                items.value['id']),
                                            builder: (context, snapshot) {
                                              if (snapshot.hasData) {
                                                return Checkbox(    // it display empty!!
                                                  tristate: true,
                                                  onChanged: null,
                                                  value: snapshot.data == "0"
                                                      ? false
                                                      : true,
                                                );
                                              } else {
                                                return Text("No data");
                                              }
                                            }),
                                        Checkbox(
                                          tristate: true,
                                          value: answers[items.value['id']] ??
                                              false,

                                          onChanged: (answer) {
                                            setState(() {
                                              answers[items.value['id']] =
                                                  answer;
                                            });
                                          },
                                          // onChanged: _onRememberMeChanged,
                                        ),
                                      ]);
                                    }).toList()