The getter 'length' was called on null

I'm trying to get data from firestore database and this code worked perfectly. But now it's not working and I'm getting this error

The getter 'length' was called on null.

Receiver: null

Tried calling: length"

this is the code getting error.

class MiddleHelpers extends ChangeNotifier {
  Widget datafav(BuildContext context, String collection) {
    return Container(
      height: 280.0,
      child: FutureBuilder(
        future: Provider.of<ManageData>(context, listen: false)
            .fetchData(collection),
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (snapshot.connectionState == ConnectionState.waiting) {
            return const Center(
              child: CircularProgressIndicator(),
            );
          } else {
            return ListView.builder(
              scrollDirection: Axis.horizontal,
              itemCount: snapshot.data.length,
              itemBuilder: (BuildContext context, int index) {
                return GestureDetector(
                  onTap: () {
                    Navigator.push(
                      context,
                      PageTransition(
                          child: DetailScreen(
                              queryDocumentSnapshot: snapshot.data[index]),
                          type: PageTransitionType.bottomToTop),
                    );
                  },
                  child: Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Container(
                      decoration: const BoxDecoration(
                        borderRadius: BorderRadius.all(
                          Radius.circular(20.0),
                        ),
                        color: Colors.white,
                        boxShadow: [
                          BoxShadow(
                            color: Colors.black26,
                            blurRadius: 5,
                            // spreadRadius: 3,
                            offset: Offset(3, 3),
                          ),
                        ],
                      ),
                    ),
                  ),
                );
              },
            );
          }
        },
      ),
    );
  }
}

fetch data method

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/cupertino.dart';

class ManageData extends ChangeNotifier {
  FirebaseFirestore firebaseFirestore = FirebaseFirestore.instance;
  Future fetchData(String collection) async {
    QuerySnapshot querySnapshot =
        await firebaseFirestore.collection(collection).get();
    return querySnapshot.docs;
  }
}

error on device

CodePudding user response：

I guess your snapshot.data is null there may be error . try adding

if (snapshot.hasError){ return Text('Something went wrong'); } inside builder

more info here https://firebase.flutter.dev/docs/firestore/usage

CodePudding user response：

It seems you are receiving no data from future and that's why you can't use .length. Also, it is necessary to make sure data existence and then use .length.

It would be better to introduce all state that can be occurred during FutureBuilder process.

Check

snapshot.connectionState if it is waiting or done. Then
if snapshot.hasError, finally else or hasData
if snapshot.hasData, also you can check data length>0 or snapshot.data!.exist then return ListView.builder

I prefer this example of firestore.

    if (snapshot.hasError) {
          return Text("Something went wrong");
        }

        if (snapshot.hasData && !snapshot.data!.exists) {
          return Text("Document does not exist");
        }

        if (snapshot.connectionState == ConnectionState.done) {

         return ListView.builder(
              scrollDirection: Axis.horizontal,
              itemCount: snapshot.data!.length,
   ///.............
        }

        return const Center(
              child: CircularProgressIndicator(),

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