The method was called on null Flutter error - flutter

Question

I have seen many people having this error in other StackOverflow posts but when I am struggling to implement the answers to other questions to my own code. The code being used is from the GitHub repository https://github.com/sbis04/events_demo.
This is how I call the method:
class _ThirdRoute extends State<ThirdRoute> {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Calendar"),
),
body: Column(
children: <Widget>[
DashboardScreen(),
],
),
);
}
}
And this is the method in another .dart file:
class DashboardScreen extends StatefulWidget {
#override
_DashboardScreenState createState() => _DashboardScreenState();
}
class _DashboardScreenState extends State<DashboardScreen> {
Storage storage = Storage();
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.white,
appBar: AppBar(
elevation: 0,
backgroundColor: Colors.white,
iconTheme: IconThemeData(
color: Colors.grey, //change your color here
),
title: Text(
'Event Details',
style: TextStyle(
fontSize: 22,
),
),
),
floatingActionButton: FloatingActionButton(
child: Icon(Icons.add),
onPressed: () {
Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => CreateScreen(),
),
);
},
),
body: Padding(
padding: const EdgeInsets.only(
left: 16.0,
right: 16.0,
),
child: Container(
child: StreamBuilder(
stream: storage.retrieveEvents(),
builder: (context, snapshot) {
if (snapshot.hasData) {
if (snapshot.data.documents.length > 0) {
return ListView.builder(
physics: BouncingScrollPhysics(),
itemCount: snapshot.data.documents.length,
itemBuilder: (context, index) {
Map<String, dynamic> eventInfo = snapshot.data.documents[index].data();
EventInfo event = EventInfo.fromMap(eventInfo);
DateTime startTime = DateTime.fromMillisecondsSinceEpoch(event.startTimeInEpoch);
DateTime endTime = DateTime.fromMillisecondsSinceEpoch(event.endTimeInEpoch);
String startTimeString = DateFormat.jm().format(startTime);
String endTimeString = DateFormat.jm().format(endTime);
String dateString = DateFormat.yMMMMd().format(startTime);
return Padding(
padding: EdgeInsets.only(bottom: 16.0),
child: InkWell(
onTap: () {
Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => EditScreen(event: event),
),
);
},
child: Column(
mainAxisAlignment: MainAxisAlignment.start,
crossAxisAlignment: CrossAxisAlignment.start,
mainAxisSize: MainAxisSize.min,
children: [
Container(
padding: EdgeInsets.only(
bottom: 16.0,
top: 16.0,
left: 16.0,
right: 16.0,
),
= child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
SizedBox(height: 10),
Text(
event.description,
maxLines: 2,
overflow: TextOverflow.ellipsis,
),
SizedBox(height: 10),
Padding(
padding: const EdgeInsets.only(top: 8.0, bottom: 8.0),
child: Text(
event.link,
),
),
SizedBox(height: 10),
Row(
crossAxisAlignment: CrossAxisAlignment.center,
children: [
Container(
height: 50,
width: 5,
),
SizedBox(width: 10),
Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Text(
dateString,
),
Text(
'$startTimeString - $endTimeString',
),
],
)
],
),
],
),
),
],
),
),
);
},
);
} else {
return Center(
child: Text(
'No Events',
),
);
}
}
return Center(
child: CircularProgressIndicator(
strokeWidth: 2,
),
);
},
),
),
),
);
}
}
If anyone would be able to give me some advice on how to solve the error:
The method 'collection' was called on null. Receiver: null. Tried calling: collection("events"). The relevant error-causing widget was DashboardScreen
EDIT This is my storage.dart file as requested in the comments:
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'calendar_client.dart';
final CollectionReference mainCollection = FirebaseFirestore.instance.collection('event');
final DocumentReference documentReference = mainCollection.doc('test');
class Storage {
Future<void> storeEventData(EventInfo eventInfo) async {
DocumentReference documentReferencer = documentReference.collection('events').doc(eventInfo.id);
Map<String, dynamic> data = eventInfo.toJson();
print('DATA:\n$data');
await documentReferencer.set(data).whenComplete(() {
print("Event added to the database, id: {${eventInfo.id}}");
}).catchError((e) => print(e));
}
Future<void> updateEventData(EventInfo eventInfo) async {
DocumentReference documentReferencer = documentReference.collection('events').doc(eventInfo.id);
Map<String, dynamic> data = eventInfo.toJson();
print('DATA:\n$data');
await documentReferencer.update(data).whenComplete(() {
print("Event updated in the database, id: {${eventInfo.id}}");
}).catchError((e) => print(e));
}
Future<void> deleteEvent({#required String id}) async {
DocumentReference documentReferencer = documentReference.collection('events').doc(id);
await documentReferencer.delete().catchError((e) => print(e));
print('Event deleted, id: $id');
}
Stream<QuerySnapshot> retrieveEvents() {
Stream<QuerySnapshot> myClasses = documentReference.collection('events').orderBy('start').snapshots();
return myClasses;
}
}