Null check operator used on null value flutter.dart

Question

The logic in the program I wrote is that it is like Twitter. There will be photo sharing and text sharing. However, there is an error in my codes and when I do not share photos, I get the error 'Null check-operator used on null value'. How can I change this?

When I do not share photos, I get the error below.

These are my codes. I would appreciate it if you could tell me the changes I need to make.

add_post_screenn

class AddPostScreen extends StatefulWidget {
  const AddPostScreen({Key? key}) : super(key: key);

  @override
  State<AddPostScreen> createState() => _AddPostScreenState();
}

class _AddPostScreenState extends State<AddPostScreen> {
  Uint8List? _file;
  final TextEditingController _descriptionController = TextEditingController();
  bool _isLoading = false;

  void postImage(
    String uid,
    String username,
    String profImage,
  ) async {
    setState(() {
      _isLoading = true;
    });
    try {
      String res = await FirestoreMethods().uploadPost(
          _descriptionController.text, _file!, uid, username, profImage);

      if (res == 'succes') {
        setState(() {
          _isLoading = false;
        });
        showSnackBar('Posted!', context);
        clearImage();
      } else {
        showSnackBar(res, context);
      }
    } catch (e) {
      showSnackBar(e.toString(), context);
    }
  }

  _selectImage(BuildContext context) async {
    return showDialog(
        context: context,
        builder: (context) {
          return SimpleDialog(
            title: const Text('Create a Post'),
            children: [
              SimpleDialogOption(
                padding: const EdgeInsets.all(20),
                child: const Text('Take a photo'),
                onPressed: () async {
                  Navigator.of(context).pop();
                  Uint8List file = await pickImage(
                    ImageSource.camera,
                  );
                  setState(() {
                    _file = file;
                  });
                },
              ),
              SimpleDialogOption(
                padding: const EdgeInsets.all(20),
                child: const Text('Chose from gallery'),
                onPressed: () async {
                  Navigator.of(context).pop();
                  Uint8List file = await pickImage(
                    ImageSource.gallery,
                  );
                  setState(() {
                    _file = file;
                  });
                },
              ),
              SimpleDialogOption(
                  padding: const EdgeInsets.all(20),
                  child: const Text('Cancel'),
                  onPressed: () {
                    Navigator.of(context).pop();
                  }),
            ],
          );
        });
  }

  void clearImage() {
    _file = null;
  }

  @override
  void dispose() {
    super.dispose();
    _descriptionController.dispose();
  }

  @override
  Widget build(BuildContext context) {
    final User user = Provider.of<UserProvider>(context).getUser;

//yorum satırı olacak
    return _file == null
        ? Center(
            child: IconButton(
              icon: const Icon(Icons.upload),
              onPressed: () => _selectImage(context),
            ),
          )

        //     //buraya kadar
        : Scaffold(
            appBar: AppBar(
              backgroundColor: mobileBackgroundColor,
              leading: IconButton(
                icon: const Icon(Icons.arrow_back),
                onPressed: clearImage,
              ),
              title: const Text('Paylaşım'),
              centerTitle: false,
              actions: [
                TextButton(
                    onPressed: () => postImage(
                          user.uid,
                          user.username,
                          user.photoUrl,
                        ),
                    child: const Text(
                      'Paylaş',
                      style: TextStyle(
                        color: Colors.blueAccent,
                        fontWeight: FontWeight.bold,
                        fontSize: 16,
                      ),
                    ))
              ],
            ),
            body: Column(
              children: [
                _isLoading
                    ? const LinearProgressIndicator()
                    : const Padding(
                        padding: EdgeInsets.only(
                          top: 0,
                        ),
                      ),
                const Divider(),
                Row(
                  mainAxisAlignment: MainAxisAlignment.spaceAround,
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: [
                    CircleAvatar(
                      backgroundImage: NetworkImage(user.photoUrl),
                    ),
                    SizedBox(
                      width: MediaQuery.of(context).size.width * 0.45,
                      child: TextField(
                        controller: _descriptionController,
                        decoration: const InputDecoration(
                          hintText: 'Hayallerini Yaz!',
                          border: InputBorder.none,
                        ),
                        maxLines: 8,
                      ),
                    ),
                    SizedBox(
                      height: 45,
                      width: 45,
                      child: AspectRatio(
                        aspectRatio: 487 / 451,
                        child: Container(
                          decoration: BoxDecoration(
                              image: DecorationImage(
                            image: MemoryImage(_file!),
                            fit: BoxFit.fill,
                            alignment: FractionalOffset.topCenter,
                          )),
                        ),
                      ),
                    ),
                    const Divider(),
                  ],
                ),
              ],
            ),
          );
  }
}

firestore_methods

class FirestoreMethods {
  final FirebaseFirestore _firestore = FirebaseFirestore.instance;

  //upload post

  Future<String> uploadPost(
    String description,
    Uint8List file,
    String uid,
    String username,
    String profImage,
  ) async {
    String res = 'Some error occurred';

    try {
      String photoUrl =
          await StorageMethods().uploadImageToStorage('posts', file, true);

      String postId = const Uuid().v1();

      Post post = Post(
        description: description,
        uid: uid,
        username: username,
        postId: postId,
        datePublished: DateTime.now(),
        postUrl: photoUrl,
        porfImage: profImage,
        likes: [],
      );

      _firestore.collection('posts').doc(postId).set(
            post.toJson(),
          );
      res = 'succes';
    } catch (err) {
      res = err.toString();
    }
    return res;
  }

  Future<void> LikePost(String postId, String uid, List likes) async {
    try {
      if (likes.contains(uid)) {
        await _firestore.collection('posts').doc(postId).update({
          'likes': FieldValue.arrayRemove([uid]),
        });
      } else {
        await _firestore.collection('posts').doc(postId).update({
          'likes': FieldValue.arrayUnion([uid]),
        });
      }
    } catch (e) {
      print(
        e.toString(),
      );
    }
  }

  Future<void> postComment(String postId, String text, String uid, String name,
      String profilePic) async {
    try {
      if (text.isNotEmpty) {
        String commentId = const Uuid().v1();
        await _firestore
            .collection('posts')
            .doc(postId)
            .collection('comments')
            .doc(commentId)
            .set({
          'profilePic': profilePic,
          'name': name,
          'uid': uid,
          'text': text,
          'commentId': commentId,
          'datePublished': DateTime.now(),
        });
      } else {
        print('Text is empty');
      }
    } catch (e) {
      print(
        e.toString(),
      );
    }
  }

storage.methods

class StorageMethods {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  final FirebaseStorage _storage = FirebaseStorage.instance;

  // firebase deposuna resim ekleme firebase deposuna resim ekleme
  Future<String> uploadImageToStorage(
      String childName, Uint8List file, bool isPost) async {
    // firebase depolama alanımıza konum oluşturma

    Reference ref =
        _storage.ref().child(childName).child(_auth.currentUser!.uid);

    if (isPost) {
      String id = const Uuid().v1();
      ref = ref.child(id);
    }

    // putting in uint8list format -> Upload task like a future but not future
    UploadTask uploadTask = ref.putData(file);

    TaskSnapshot snap = await uploadTask;
    String downloadUrl = await snap.ref.getDownloadURL();
    return downloadUrl;
  }
}

Answer 1

The error cause comes from the _file! . When posting only text there is going to be no _file as it's going to be null .

To fix this change the references from _file! to just _file and the FirestoreMethods.uploadPost should be the following:

FirestoreMethods.uploadPost - Change file to be nullable and check for null.

  Future<String> uploadPost(
    String description,
    Uint8List? file,                       // <- Here
    String uid,
    String username,
    String profImage,
  ) async {
    String res = 'Some error occurred';

    try {
      String photoUrl = file != null       // <- Here
          ? await StorageMethods().uploadImageToStorage('posts', file, true)
          : '';

      String postId = const Uuid().v1();

      Post post = Post(
        description: description,
        uid: uid,
        username: username,
        postId: postId,
        datePublished: DateTime.now(),
        postUrl: photoUrl,
        porfImage: profImage,
        likes: [],
      );

      _firestore.collection('posts').doc(postId).set(
            post.toJson(),
          );
      res = 'succes';
    } catch (err) {
      res = err.toString();
    }
    return res;
  }

_AddPostScreenState.postImage - Change _file! to just _file

  void postImage(
    String uid,
    String username,
    String profImage,
  ) async {
    setState(() {
      _isLoading = true;
    });
    try {
      String res = await FirestoreMethods().uploadPost(
          _descriptionController.text, _file, uid, username, profImage);

      if (res == 'succes') {
        setState(() {
          _isLoading = false;
        });
        showSnackBar('Posted!', context);
        clearImage();
      } else {
        showSnackBar(res, context);
      }
    } catch (e) {
      showSnackBar(e.toString(), context);
    }
  }

Answer 2

I think you are getting error in this line

 Reference ref =
        _storage.ref().child(childName).child(_auth.currentUser!.uid);

Here the current user maybe null. To fix this error for now you can use

Reference ref =
        _storage.ref().child(childName).child(_auth.currentUser?.uid);
//Note instead of using ! I have used ? which means currentUser can be null. ! means you are marking that its not null and value is unknown

but please note that you are receiving _auth.currentUser as null. Please check those codes

Answer 3

I get this error. Returns the current [User] if they are currently signed-in, or null if not.

Answer 4

I get the same error on _file. On this line: backgroundImage: MemoryImage(_file,),