Flutter: Unable to read data from firebase. The method '[]' was called on null error

问题

I am trying to read the data from firebase after scanning a barcode.

This is how it should appear, but instead of barcode, it should display name and price from the database (https://m.imgur.com/gallery/lEFJZ0Q)

Code:

class ListTileModel {
  String barcode;
  ListTileModel(this.barcode);
}

the below code is inside the stateful widget

List<ListTileModel> _items = [];
String barcode = "";

  void _add() {
    _items.add(ListTileModel(barcode));
      setState(() {});
  }


  @override
  void initState(){
    super.initState();
  }

StreamBuiler Used:

new Container(

              child: ListView(
                  children: _items
                      .map((item) => StreamBuilder(
                  stream: FirebaseDatabase.instance.reference().child('prd').child(item.barcode).onValue,
                  builder: (context, snap) {
                    print(item.barcode);
                    if(snap.hasData){
                      DataSnapshot snapshot = snap.data.snapshot;
                      Map<dynamic, dynamic> itm = snapshot.value;
                      return snap.data.snapshot.value == null
                          ? SizedBox()
                          : ListView.builder(
                        shrinkWrap: true,
                        scrollDirection: Axis.vertical,
                        itemCount: 1,
                        itemBuilder: (context, index) {
                          return Row(
                            children: <Widget>[

                              ConstrainedBox(

                                child: Container(

                                  child: Text(itm[index]['name'],style: TextStyle(fontSize: 20),),
                                ),
                              ),
                              SizedBox(width: 100,),
                              ConstrainedBox(

                                child: Container(

                                  child: Text(itm[index]['price'].toString(),style: TextStyle(fontSize: 20),),
                                ),
                              ),
                            ],
                          );
                        },
                      );
                    }
                    else {
                      return   Center(child: CircularProgressIndicator());
                    }
                  }
              ),
                  ).toList()
              ),
            ),

The Barcode Scan code:

widget:

floatingActionButton: FloatingActionButton(
          onPressed: scan,
        child: Icon(Icons.add_shopping_cart),

      ),

scan() :

Future scan() async{
    try{
      String barcode = await BarcodeScanner.scan();
        setState(() {
          this.barcode = barcode;
        });
      _add();
    }on PlatformException catch(e) {
      if(e.code == BarcodeScanner.CameraAccessDenied){
        setState(() {
          this.barcode = 'Access Denied';
        });
      }
    }

I'm am getting following Error:

The following NoSuchMethodError was thrown building: The method '[]' was called on null. Receiver: null Tried calling:

回答1:

Please try this and let me know if it fixes your problem you need to change

Text(itm[index]['name'],style: TextStyle(fontSize: 20),),

Text(itm[index]['price'].toString(),style: TextStyle(fontSize: 20),),

To

Text(itm['det']['name'],style: TextStyle(fontSize: 20),),

Text(itm['det']['price'].toString(),style: TextStyle(fontSize: 20),),

Let me know if this works for you. I believe the problem is the index also. right now your saying.

来源：https://stackoverflow.com/questions/61284432/flutter-unable-to-read-data-from-firebase-the-method-was-called-on-null-e