Null check operator used on null value flutter.dart - flutter

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