Dart Null Coalescing not returning default value - firebase-realtime-database

Question

I have a list of staff 4 of whom are external consultants so they have no department in our Firebase Realtime Database.
In a FirebaseAnimatedList I am listing staff in an employee directory.
In my model I am trying to deal with those null values to suppress the error:
A non-null String must be provided to a Text widget. flutter:
'package:flutter/src/widgets/text.dart': flutter: Failed assertion:
line 253 pos 10: 'data != null'
I have used the following to no avail:
department = snapshot.value['department'] ?? 'Managment',
department = snapshot.value['department'] ??= 'Managment',
department = snapshot.value['department']
? snapshot.value['department']
: 'Management',
Any suggestions for this Flutter beginner is appreciated.
UPDATED with solution -- this is my entire model:
import 'package:firebase_database/firebase_database.dart';
class Staff {
String key;
String avatar;
String badge;
int dbId;
String department;
String directLine;
String email;
String extension;
String firstName;
bool isOnline;
String lastName;
String location;
String name;
String status;
String title;
String uid;
Staff(
this.key,
this.avatar,
this.badge,
this.dbId,
this.department,
this.directLine,
this.email,
this.extension,
this.firstName,
this.isOnline,
this.lastName,
this.location,
this.name,
this.status,
this.title,
this.uid);
Staff.fromSnapshot(DataSnapshot snapshot)
: key = snapshot.key,
avatar = "https://files.uaminc.com/images/avatars/" +
snapshot.value['avatar'],
badge = snapshot.value['badge'],
dbId = snapshot.value['dbId'],
department = snapshot.value.containsKey('department')
? snapshot.value['department']
: 'Management',
directLine = snapshot.value['directLine'],
email = snapshot.value['email'],
extension = snapshot.value['extension'],
firstName = snapshot.value['firstName'],
isOnline = snapshot.value['isOnline'],
lastName = snapshot.value['lastName'],
location = snapshot.value['location'],
name = snapshot.value['name'],
status = snapshot.value['status'],
title = snapshot.value.containsKey('title')
? snapshot.value['title']
: 'Management',
uid = snapshot.value['uid'];
toJson() {
return {
"key": key,
"avatar": avatar,
"badge": badge,
"dbId": dbId,
"department": department,
"directLine": directLine,
"email": email,
"extension": extension,
"firstName": firstName,
"isOnline": isOnline,
"lastName": lastName,
"location": location,
"name": name,
"status": status,
"title": title,
"uid": uid,
};
} // end method
}