error: The instance member 'widget' can't be accessed in an initializer

I can't work out what I'm doing wrong such that widget.event on line 15 is showing the mentioned error. I believe initState can fix it but I can't for the life of me work out how I implement it.

class _EventDetailsScreenState extends State<EventDetailsScreen> {
  final _database = FirebaseDatabase.instance;
  final _eventRef = FirebaseDatabase.instance.reference().child('events/' + widget.event['eventId']);

The full code is below.

import 'package:flutter/material.dart';
import 'package:firebase_database/firebase_database.dart';

class EventDetailsScreen extends StatefulWidget {
  final event;

  EventDetailsScreen({Key? key, this.event}) : super(key: key);

  @override
  _EventDetailsScreenState createState() => _EventDetailsScreenState();
}

class _EventDetailsScreenState extends State<EventDetailsScreen> {
  final _database = FirebaseDatabase.instance;
  final _eventRef = FirebaseDatabase.instance.reference().child('events/' + widget.event['eventId']);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.event['eventName']),
      ),
      body: StreamBuilder(
        stream: _eventRef.onValue,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            final event = snapshot.data!.snapshot.value as Map<String, dynamic>;
            return Padding(
              padding: const EdgeInsets.all(16.0),
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  Text('Event Name: ' + event['eventName']),
                  Text('Event Date: ' + event['eventDate']),
                  Text('Event Time: ' + event['eventTime']),
                  Text('Event Location: ' + event['eventLocation']),
                  Text('Invited Users: ' + event['invitedUsers'].toString()),
                ],
              ),
            );
          } else {
            return Center(child: Text('Loading'));
          }
        },
      ),
    );
  }
}

I tried to implement initState but I can't seem to get it right, and nay adjustments I make seem to lead to more errors. It is my last error and it's driving me crazy!

As an explanation: you can't access the widget property inside of the constructor, or for initializing the members of the state object, because the state object is only mounted (connected to the widget) right before the initState method is called.

  late final DatabaseReference _eventRef;

  @override
  void initState() {
    super.initState();
    _eventRef = FirebaseDatabase.instance.reference().child('events/' + widget.event['eventId']);
  }

The other answer from @jabguru is also an option with the inline late final initilizer, because the right side (the initialization) will only be called when the member variable is accessed the first time.

The error is on this line of code

final _eventRef = FirebaseDatabase.instance.reference().child('events/' + widget.event['eventId']);

You can't use "widget" like that, but you can simply fix this by adding the "late" keyword

late final _eventRef = FirebaseDatabase.instance.reference().child('events/' + widget.event['eventId']);