Error with my project - The method 'collection' was called on null [duplicate] - flutter

Question

This question already has answers here:
What is a NoSuchMethod error and how do I fix it?
(2 answers)
Closed 2 years ago.
Good,
I have a problem with my application, since it generates an error when pressing the add to cart button, it generates an error and it is not added in firebase, I already checked that the variables have their name correctly, however I cannot find how solve this error, I appreciate all the help you can give me to solve this error!
void checkItemInCart(String shortInfoAsID, BuildContext context)
{
EcommerceApp.sharedPreferences.getStringList(EcommerceApp.userCartList).contains(shortInfoAsID)
? Fluttertoast.showToast(msg: "El artículo ya existe en el carrito")
: addItemToCart(shortInfoAsID, context);
}
addItemToCart(String shortInfoAsID, BuildContext context) {
List tempCartList = EcommerceApp.sharedPreferences.getStringList(EcommerceApp.userCartList);
tempCartList.add(shortInfoAsID);
EcommerceApp.firestore.collection(EcommerceApp.collectionUser)
.document(EcommerceApp.sharedPreferences.getString(EcommerceApp.userUID))
.updateData({
EcommerceApp.userCartList: tempCartList,
}).then((v){
Fluttertoast.showToast(msg: "Artículo añadido al carrito");
EcommerceApp.sharedPreferences.setStringList(EcommerceApp.userCartList, tempCartList);
Provider.of<CartItemCounter>(context, listen: false).displayResult();
});
}
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:e_shop/Store/storehome.dart:283:26)
2 checkItemInCart (package:e_shop/Store/storehome.dart:276:9)
3 sourceInfo.<anonymous closure> (package:e_shop/Store/storehome.dart:246:27)
4 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:993:19)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#696c2
debugOwner: GestureDetector
state: ready
won arena
finalPosition: Offset(338.5, 338.3)
finalLocalPosition: Offset(32.5, 27.8)
button: 1
sent tap down
====================================================================================================

Answer 1

Declare this:
List tempCartList = List<>();
Then do this :
void checkItemInCart(String shortInfoAsID, BuildContext context)
{
EcommerceApp.sharedPreferences.getStringList(EcommerceApp.userCartList).contains(shortInfoAsID)
? Fluttertoast.showToast(msg: "El artículo ya existe en el carrito")
: addItemToCart(shortInfoAsID, context);
}
addItemToCart(String shortInfoAsID, BuildContext context) {
tempCartList = EcommerceApp.sharedPreferences.getStringList(EcommerceApp.userCartList);
tempCartList.add(shortInfoAsID);
EcommerceApp.firestore.collection(EcommerceApp.collectionUser)
.document(EcommerceApp.sharedPreferences.getString(EcommerceApp.userUID))
.updateData({
EcommerceApp.userCartList: tempCartList,
}).then((v){
Fluttertoast.showToast(msg: "Artículo añadido al carrito");
EcommerceApp.sharedPreferences.setStringList(EcommerceApp.userCartList, tempCartList);
Provider.of<CartItemCounter>(context, listen: false).displayResult();
});
}