Expected a value of type 'String', but got one of type 'Null' StreamBuilder<QuerySnapshot<Object?>> - flutter

Question

I trying create a contact list to be displayed on my screen as a user edit of delete his details. I got this error and UI doesn't show anything. The error is : Expected a value of type 'String', but got one of type 'Null'
here is where error happens:
---dart
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
class contactlist extends StatefulWidget {
const contactlist({ Key? key }) : super(key: key);
#override
State<contactlist> createState() => _contactlistState();
}
class _contactlistState extends State<contactlist> {
final Stream<QuerySnapshot>_myUserContacts =
FirebaseFirestore.instance.collection('userContact').snapshots();
#override
Widget build(BuildContext context) {
TextEditingController _nameFieldcntroler = TextEditingController();
TextEditingController _phoneNumFieldcntroler = TextEditingController();
TextEditingController _EmailFieldcntroler = TextEditingController();
TextEditingController _AgeFieldcntroler = TextEditingController();
void _delete(docId){
FirebaseFirestore.instance
.collection("userContact")
.doc(docId)
.delete()
.then((value) => print("deleted"));
}
void _update(data){
var collection = FirebaseFirestore.instance.collection("userContact");
_nameFieldcntroler.text = data["names"];
_phoneNumFieldcntroler.text = data["phoneNumber"];
_EmailFieldcntroler.text = data["email"];
_AgeFieldcntroler.text = data["age"];
showDialog(context: context,
builder: (_) => AlertDialog(
title: Text("Update"),
content: Column(
mainAxisSize: MainAxisSize.min,
children: [
TextField(
controller:_nameFieldcntroler,
),
TextField(
controller:_phoneNumFieldcntroler,
),
TextField(
controller:_EmailFieldcntroler,
),
TextField(
controller:_AgeFieldcntroler,
),
TextButton(
onPressed: (){
collection.doc(data["doc_Id"])
.update({
"names": _nameFieldcntroler.text,
"phoneNumber": _phoneNumFieldcntroler.text,
"email": _EmailFieldcntroler.text,
"age": _AgeFieldcntroler.text,
});
Navigator.pop(context);
},
child: Text("Update")),
]),
)
);
}
return StreamBuilder(
stream: _myUserContacts,
builder: (BuildContext context,
AsyncSnapshot<QuerySnapshot<Object?>> snapshot){
if (snapshot.hasError){
return const Text("Something went wrong");
}
if(snapshot.connectionState == ConnectionState.waiting){
return const Center(child: CircularProgressIndicator());
}
if(snapshot.hasData){
return Row(
children: [
Expanded(
child: SizedBox(
height: (MediaQuery.of(context).size.height),
width: (MediaQuery.of(context).size.width),
child: ListView(
children: snapshot.data!.docs
.map((DocumentSnapshot documentSnapshot) {
Map<String, dynamic> data = documentSnapshot.data()! as Map<
String, dynamic>;
return Column(
children: [
Card(
child: Column(
children:[
ListTile(
title: Text(data['names']),
subtitle:Text(data['phoneNumber']),
),
ButtonTheme(
child: ButtonBar(
children:[
OutlineButton.icon(
onPressed:(){
_update(data);
},
icon: Icon(Icons.edit),
label: Text("Edit"),
),
OutlineButton.icon(
onPressed:(){
_delete(data["doc_Id"]);
},
icon: Icon(Icons.remove),
label: Text("Delete"),
)
],
),
),
],
)
)
],
);
}).toList(),
),
)
)
],
);
}else{
return(Text("No data"));
}
},
);
}
}
---dart
---dart
The following TypeErrorImpl was thrown building StreamBuilder<QuerySnapshot<Object?>>(dirty, dependencies: [MediaQuery], state: _StreamBuilderBaseState<QuerySnapshot<Object?>, AsyncSnapshot<QuerySnapshot<Object?>>>#1efa0):
Expected a value of type 'String', but got one of type 'Null'
The relevant error-causing widget was
StreamBuilder<QuerySnapshot<Object?>>
---dart