Exception caught by gesture The following NoSuchMethodError was thrown while handling a gesture

i'm newbie with flutter i'm trying to add product to cart but when i click to add button it shown that collection method is called on null the function i'm using to add the product to the user cart :

void checkItemInCart(String shortInfoAsID, BuildContext context) {
      Constants.sharedPreferences
              .getStringList(Constants.userCartList)
              .contains(shortInfoAsID)
          ? Fluttertoast.showToast(msg: "Item is already in Cart.")
          : addItemToCart(shortInfoAsID, context);
    }
    
    addItemToCart(String shortInfoAsID, BuildContext context) {
      List tempCartList =
          Constants.sharedPreferences.getStringList(Constants.userCartList);
      tempCartList.add(shortInfoAsID);
    
      Constants.firestore
          .collection(Constants.collectionUser)
          .document(Constants.sharedPreferences.getString(Constants.userUID))
          .updateData({
        Constants.userCartList: tempCartList,
      }).then((v) {
        Fluttertoast.showToast(msg: "Item Added to Cart Successfully.");
    
        Constants.sharedPreferences
            .setStringList(Constants.userCartList, tempCartList);
    
        Provider.of<CartItemCounter>(context, listen: false).displayResult();
      });
    }

Also i have the constants file which is :

class Constants {
      static const String appName = 'App name';
    
      static SharedPreferences sharedPreferences;
      static FirebaseUser user;
      static FirebaseAuth auth;
      static Firestore firestore;
    
      static String collectionUser = "users";
      static String collectionOrders = "orders";
      static String userCartList = 'userCart';
      static String subCollectionAddress = 'userAddress';
    
      static final String userName = 'name';
      static final String userEmail = 'email';
      static final String userPhotoUrl = 'photoUrl';
      static final String userUID = 'uid';
      static final String userAvatarUrl = 'url';
    
      static final String addressID = 'addressID';
      static final String totalAmount = 'totalAmount';
      static final String productID = 'productIDs';
      static final String paymentDetails = 'paymentDetails';
      static final String orderTime = 'orderTime';
      static final String isSuccess = 'isSuccess';
    }

the error i'm getting is :

════════ Exception caught by gesture ═══════════════════════════════════════════
    The following NoSuchMethodError was thrown while handling a gesture:
    The method 'collection' was called on null.
    Receiver: null
    Tried calling: collection("users")
    
    When the exception was thrown, this was the stack
    #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
    #1      addItemToCart
    package:shipit/Store/storehome.dart:371
    #2      checkItemInCart
    package:shipit/Store/storehome.dart:362

i hope you can help me thank you in advance appreciate it.

This happens because Constants.firestore is null. You need to set Constants.firestore to contain an instance of Firestore. For example, you could do this:

// in lib/main.dart

void main() {
  WidgetsFlutterBinding.ensureInitialized();  // make sure plugins are initialized
  Constants.firestore = Firestore.instance;  // Constants.firestore is not null after this line
  runApp(MyApp());  // launch the app
}

If you set your main() method to look like this, Constants.firestore will never be null when you try to use it in your widgets/state management.

You may also want to consider doing this for the other things in Constants too (e.g. FirebaseAuth, SharedPreferences etc)