I want to do a to do list app. I use Sqflite in my project.

Widget

  return FutureBuilder(
    future: databaseTrans.getTables(),
    builder: (context, snapshot) {
      if (snapshot.data == null) {
        return taskBookErrorWidget(context);
      } else {
        return GridView.count(
          crossAxisCount: 3,
          children: List.generate(snapshot.data!.length, (i) {
                return InkWell(
                  onTap: () {},
                  child: Container(
                    width: 300,
                    height: 300,
                    margin: EdgeInsets.all(10),
                    alignment: Alignment.center,
                    decoration: BoxDecoration(
                      color: Colors.white10,
                      borderRadius: BorderRadius.circular(30),
                    ),
                    child: Text(
                        snapshot.data![i].name.toString(),
                        style: TextStyle(color: Colors.white)),
                  ),
                );
          }),
        );
      }
    },
  );
}

Errors

The following _CastError was thrown building FutureBuilder<List<taskBooksDataModel>>(state: _FutureBuilderState<List<taskBooksDataModel>>#da7bd):
Null check operator used on a null value

The relevant error-causing widget was: 
  FutureBuilder<List<taskBooksDataModel>> FutureBuilder:file:///C:/Users/Administrator/Desktop/todolist/lib/screens/taskBooksScreen/taskBookWidget.dart:16:12
When the exception was thrown, this was the stack: 
#0      _SnackBarState.initState (package:flutter/src/material/snack_bar.dart:391:21)
#1      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:5015:57)
#2      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4853:5)
#3      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3863:16)
#4      Element.updateChild (package:flutter/src/widgets/framework.dart:3586:20)
#5      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4904:16)

Database Function

Future<List<taskBooksDataModel>> getTables() async {
    Database db = await database;

    var result = await db.query("sqlite_master");
    return taskBooksDataModel.convertToTaskBookDataModel(result);
  }

What should i do ? I want to read datas from table and write them into Text() on FutureBuilder. I have two tables. One of them includes task books' names.

CodePudding user response：

try checking ConnectionState first. then you should check data of snapshot.

if (snapshot.connectionState == ConnectionState.waiting){
        return CircularProgressIndicator();

}

CodePudding user response：

That's basically means that you've used '!' in a place where variable is null. Check to see if you returned null from your API. Also take a look at docs to see how to check if snapshot has data

CodePudding user response：

Code in your sample says that data in snapshot.data![i].name.toString() is never null but during execution it turns out sometimes it is null. Add some condition for proper handling input data in Text Widget. E.G.

Text(
    snapshot.data != null
        ? snapshot.data![i].name.toString()
        : '',
        style: TextStyle(color: Colors.white),
    ),
),