A split second error on StreamBuilder Flutter - flutter

Question

I'm trying to write full app for the first time as a beginner. I'm using getx in this app too.
I'm taking a split second RangeError (index): Invalid value: Valid value range is empty: 0 while using StreamBuilder. It is working fine after that split second red error page. How I am supposed to deal with this, should I use something related with getx to stream the data?
This is my StreamBuilder:
StreamBuilder(
stream: stream,
builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
if (snapshot.hasError) {
return const Center(
child: CircularProgressIndicator(),
);
} else if (snapshot.hasData) {
List<KTCardItem> collectionList = filterByTag(snapshot.data!.docs);
return PageView.builder(
controller: PageController(keepPage: true),
scrollDirection: Axis.vertical,
itemCount: collectionList.length,
itemBuilder: (BuildContext context, index) {
Future.delayed(const Duration(seconds: 3));
return NFTCardView(
index: index,
isFavorite: isFavoritedByUser(index),
ktCardItem: collectionList[index],
onFavChanged: () {
onFavoriteChanged(collectionList[index].eventId ?? "", index);
},
);
},
);
} else {
return const Center(child: CircularProgressIndicator());
}
// filter the list by choice of tag
},
);
I'm guessing that cause of this problem is the filterByTag method that I do before the return. filterByTag method:
List<KTCardItem> filterByTag(List<QueryDocumentSnapshot> snapshot) {
List<KTCardItem> collectionList = [];
for (var document in snapshot) {
Map<String, dynamic> data = document.data()! as Map<String, dynamic>;
if (data["tags"].contains(_tag.value)) {
collectionList.add(KTCardItem.fromMap(data));
}
}
return collectionList;
}