Flutter SQLite database , getter 'length' was called on null error, while trying to showing all rows in Database as listview

Question

I recently started using Flutter for app development. I am trying to make a simple notepad app from scratch as part of the learning assignment. can anyone help me where I am making the mistake or if I am missing any fundamental concept? Thanks in advance, below are the details of the issue.

I wrote a database_helper class and a function to show all elements in the database as a list.

//code from Database_Helper.dart
//get the total number of rows in DB.
 Future<int> getCount() async {
    Database db = await this.database;
    List<Map<String, dynamic>> x =
        await db.rawQuery('SELECT COUNT (*) from $ideaTable');
    int result = Sqflite.firstIntValue(x);
    return result;
  }

//get total rows in db as <list <map>>
  Future<List<Map<String, dynamic>>> getIdeaMapList() async {
    Database db = await this.database;

    var result = await db.rawQuery('SELECT * FROM $ideaTable ');
    return result;
  }
//converting list<map> to list<Ideas> // Ideas being defined class.
  Future<List<Idea>> getIdeaList() async {
    var ideaMapList = await getIdeaMapList(); // Get 'Map List' from database
    int count =
        ideaMapList.length; // Count the number of map entries in db table

    List<Idea> ideaList = List<Idea>();
    // For loop to create a 'todo List' from a 'Map List'
    for (int i = 0; i < count; i++) {
      ideaList.add(Idea.fromMapObject(ideaMapList[i]));
    }

    return ideaList;
  }
}

the database helper object is created in main file and list is shown as vertical Listview using ShowIdea class as shown below.

class _ShowideasState extends State<Showideas> {
  DatabaseHelper _databaseHelper = new DatabaseHelper();
  List<Idea> listideas;
  @override
  Widget build(BuildContext context) {
    debugPrint('micheal jackson:  listideaslength');

    if (listideas == null) {
      updatelistideas();
    }
    int j = listideas.length;
    debugPrint('micheal jackson: $j listideaslength');

    return Container(
      child: ListView.builder(
          itemCount: listideas.length,
          itemBuilder: (BuildContext context, int position) {
            return Card(
              color: Colors.green[200],
              elevation: 2.0,
              child: Column(
                children: <Widget>[
                  Text(this.listideas[position].iTitle),
                  Text(this.listideas[position].iText),
                  Text(this.listideas[position].date),
                ],
              ),
            );
          }),
    );
  }

  void updatelistideas() async {
    listideas = await _databaseHelper.getIdeaList();
    int i = listideas.length;
    debugPrint('ideaslength: $i listideaslength');
  }
}

the following logs

2020-08-23 20:36:51.866 24126-24169/com.example.myprojet01 I/flutter: micheal jackson:  listideaslength
2020-08-23 20:36:51.868 24126-24169/com.example.myprojet01 I/flutter: movieTitle: get database list
2020-08-23 20:36:52.358 24126-24169/com.example.myprojet01 I/flutter: idea list length in db: 5 
2020-08-23 20:36:52.358 24126-24169/com.example.myprojet01 I/flutter: ideaslength: 5 listideaslength

but the widget is not rendered and shows a red screen with error 'getter length was called on null'.

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building Showideas(dirty, state: _ShowideasState#193ad):
The getter 'length' was called on null.
Receiver: null
Tried calling: length
The relevant error-causing widget was:
  Showideas

if the app is hot reloaded without any changes. The following logs add up and the screen turns white and is overflowed.

2020-08-23 20:36:56.041 24126-24169/com.example.myprojet01 I/flutter: micheal jackson:  listideaslength
2020-08-23 20:36:56.041 24126-24169/com.example.myprojet01 I/flutter: micheal jackson: 5 listideaslength

Lapa Ny Aina Tanjona · Accepted Answer · 2020-08-23 15:30:27Z

1

Try the code below :

void updatelistideas() async {
    final ideas = await _databaseHelper.getIdeaList();
    setState(() {
      listideas = ideas;
    });
    int i = listideas.length;
    debugPrint('ideaslength: $i listideaslength');
  }

and change this :

itemCount: listideas.length,

to

itemCount: null == listideas ? 0 : listideas.length,

edited Aug 23, 2020 at 15:30

answered Aug 23, 2020 at 15:18

Lapa Ny Aina Tanjona

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2 Comments

bscharan Over a year ago

Hi @Lapa Ny Aina Tanjona, Thanks for your answer. getter length error is fixed. can you please explain me why this code works.

Lapa Ny Aina Tanjona Over a year ago

Sure! At the beginning, listideas is null,. so if it is null we call the updatelistideas () function and we have to do a setState to rebuild our widget (Showideas) again with the new value of the listideas. Please consider accepting it if my answer solve your issue :)

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