error: the method [] was called on null. tried calling ()[merchantId] - flutter

Question

I am trying to get the list of products by shop from the firestore. However, getting the following error:
I/flutter ( 7294): NoSuchMethodError: The method '[]' was called on null.
I/flutter ( 7294): Receiver: null
I/flutter ( 7294): Tried calling: []("merchantId")
I am using FutureBuilder to get shop data and StreamBuilderWrapper to get the products:
class Shop extends StatelessWidget {
Widget getProductsAndImages(BuildContext context, String outletID) {
return StreamBuilderWrapper(
shrinkWrap: true,
stream: productsRef.doc(outletID).collection('products')
.orderBy('dateTime', descending: true).snapshots(),
itemBuilder: (_, DocumentSnapshot snapshot) {
ProductModel posts = ProductModel.fromJson(snapshot.data() as Map<String, dynamic>);
return posts.productId != null
? Padding(
padding: const EdgeInsets.only(bottom: 12.0),
child:
ProductItem(productName: posts.proName!, productPrice: posts.price!,
count: posts.count!, images: posts.images!,
category: posts.category!, productDescription: posts.productDescription!,
productId: posts.productId!),
)
: Container(
child: Center(
child: Text('No products'),
),
);
});
}
#override
Widget build(BuildContext context) {
final outletData = Provider.of<OutletData>(context);
final outletId = ModalRoute.of(context)!.settings.arguments as String;
print('outlet id--->>> $outletId');
return Scaffold(
appBar: appHeader(context),
backgroundColor: color2,
extendBodyBehindAppBar: true,
body: Stack(
children: <Widget>[
Container(
decoration: const BoxDecoration(
image: DecorationImage(image: AssetImage("assets/images/background.jpg"),
fit: BoxFit.cover,),
),
),
FutureBuilder(
future: outletData.getOutletById(outletId),
builder: (ctx, AsyncSnapshot snapshot) {
if (snapshot.connectionState == ConnectionState.waiting ||
snapshot.data == null) {
return
Center(
child: Text("Юм олдсонгүй"),
);
}
return snapshot.data.length == 0
? Center(
child: Text("Бүтээгдэхүүн байхгүй"),
)
: SingleChildScrollView(
child: Column(
crossAxisAlignment: CrossAxisAlignment.center,
children: [
ShopHeader(
outletName: snapshot.data()[outletName],
outletImage: snapshot.data()[outletImg],
location: snapshot.data()[outletLocation],
description: snapshot.data()[outletIntro],
phoneNum: snapshot.data()[outletPhone],
merchantId: snapshot.data()[merchantId],
outletId: outletId,
outletRating: snapshot.data()['outletRating'],
),
Padding(
padding: const EdgeInsets.all(15.0),
child: Text(
"Recommended products:",
style: TextStyle(
fontWeight: FontWeight.bold,
fontSize: 17,
color: Colors.black54,
),
),
),
ListView.builder(
padding: EdgeInsets.zero,
shrinkWrap: true,
physics: NeverScrollableScrollPhysics(),
itemBuilder: (ctx, index) {
return
getProductsAndImages(context, outletId);
},
),
],
),
);
},
)
]));
}
}
my getOutletById method is in the provider class:
Future<Map<String, dynamic>> getOutletById(String outId) async {
var outletsById;
final Map<String, dynamic> idOutlet = {};
_outletMerchant.clear();
try {
outletsById =
await firestore.collection(outletsCollection).doc(outId).get();
var outletResponse = outletsById.data();
var outletMerchantData = await firestore
.collection(allUserCollection)
.doc(outletResponse[merchantId])
.get();
var merchantResponse = outletMerchantData.data();
_outletMerchant.addAll(merchantResponse!);
print(_outletMerchant);
idOutlet.addAll(outletResponse);
print(merchantResponse[merchantId]);
} catch (err) {
print(err);
}
return idOutlet;
}
print() method prints nothing here my firestore database is as follows:
You can see that there is the merchantId for the outlets collection. where is the error here?