Flutter – Handling errors for firestore and firebase storage

I am new to Flutter and Firebase. I am stuck on how to handle exceptions. For example, if there’s no internet or if something went wrong, I want to show a Snackbar displaying that error message. What I am trying to do is:

1. get the user to input some data and pictures
2. Upload that picture to Firebase Storage and retrieve the URL
3. Upload data input by user and URL to Firestore

4.Catch any errors and display on a Snackbar

To begin,
When I click on the save button it will call this function:

void _saveItem() async {

    if (_formKey.currentState!.validate()) {
      setState(() {
        _isSaving = true;
      });
      _formKey.currentState!.save();
      final urlLink = await uploadImage(_selectedImage);

      createListing(
        itemName: _itemName!,
        urlLink: urlLink!,
        mainCat: _chosenMainCategory!,
        subCat: _chosenSubCategory!,
        dietaryInfo: _chosenDietaryOption!,
        addInfo: _additionalInfo!,
        expDate: _chosenDate!,
        lat: _lat!,
        lng: _lng!,
        address: _address!,
      );
      Navigator.pop(context);
    }
  }

Which will then call uploadImage to retrieve the image URL:

Future<String> uploadImage(File file) async {
    String fileName = DateTime.now().millisecondsSinceEpoch.toString();
    firebase_storage.Reference ref = firebase_storage.FirebaseStorage.instance
        .ref()
        .child('listingImages/$fileName');

    await ref.putFile(file);

    String imageUrl = await ref.getDownloadURL();
    return imageUrl;
  }

Then it will call createListing to upload the document onto Firestore:

Future<void> createListing({
    required String itemName,
    required String urlLink,
    required String mainCat,
    required String subCat,
    required String dietaryInfo,
    required String addInfo,
    required DateTime expDate,
    required double lat,
    required double lng,
    required String address,
  }) async {
    final docListing = FirebaseFirestore.instance.collection('Listings').doc();

    final user = FirebaseAuth.instance.currentUser!;

    final listing = Listing(
      id: docListing.id,
      itemName: itemName,
      image: urlLink,
      mainCategory: mainCat,
      subCategory: subCat,
      dietaryNeeds: dietaryInfo,
      additionalNotes: addInfo,
      expiryDate: expDate,
      lat: lat,
      lng: lng,
      address: address,
      isAvailable: true,
      userId: user.email.toString(),
      userName: user.displayName.toString(),
    );

    final json = listing.toJson();
    await docListing.set(json);
  }

I don’t know where or how I am supposed to handle errors or exceptions.

I have tried using try catch on everything that has ‘await’ but it doesnt seem to timeout and the snackbar was never shown on screen.

This is my utils class i use for displaying errors on snackbars.

class Utils {
  static final messengerKey = GlobalKey<ScaffoldMessengerState>();

  static showSnackBar(String? text) {
    if (text == null) return;

    final snackBar = SnackBar(content: Text(text), backgroundColor: Colors.red);

    messengerKey.currentState!
      ..removeCurrentSnackBar()
      ..showSnackBar(snackBar);
  }
}

The way i use it to display an error e on screen:

Utils.showSnackBar(e.message);

Would like to get some comments on my edit, i am not sure whether this would suffice just by adding a try catch when calling saveItem function:

void _saveItem() async {
    if (_selectedImage == null) {
      showDialog(
        context: context,
        builder: (ctx) => AlertDialog(
          title: const Text(
            'Picture missing',
            textAlign: TextAlign.center,
          ),
          content: const SizedBox(
            height: 16,
            width: 32,
            child: Center(
              child: Text('Add a picture'),
            ),
          ),
          actions: [
            TextButton(
              onPressed: () {
                Navigator.pop(ctx);
              },
              child: const Text('ok'),
            ),
          ],
        ),
      );
      return;
    }
    if (_address == null) {
      showDialog(
        context: context,
        builder: (ctx) => AlertDialog(
          title: const Text(
            'Address missing',
            textAlign: TextAlign.center,
          ),
          content: const SizedBox(
            height: 16,
            width: 32,
            child: Center(
              child: Text('Click on get address'),
            ),
          ),
          actions: [
            TextButton(
              onPressed: () {
                Navigator.pop(ctx);
              },
              child: const Text('ok'),
            ),
          ],
        ),
      );
      return;
    }
    if (_formKey.currentState!.validate()) {
      setState(() {
        _isSaving = true;
      });
      _formKey.currentState!.save();
      try {
        final urlLink = await uploadImage(_selectedImage);

        createListing(
          itemName: _itemName!,
          urlLink: urlLink!,
          mainCat: _chosenMainCategory!,
          subCat: _chosenSubCategory!,
          dietaryInfo: _chosenDietaryOption!,
          addInfo: _additionalInfo!,
          expDate: _chosenDate!,
          lat: _lat!,
          lng: _lng!,
          address: _address!,
        );
      } catch (e) {
        Utils.showSnackBar('error: $e');
      }
      Navigator.pop(context);
    }
  }