Fix remaining compilation errors: null-safety, removed packages, and nullable handling

- Fix AppState: remove removed connectivity package, fix imports (latlong2), make nullable fields optional
- Fix User model: use const empty strings in const constructor instead of null
- Fix model builders: add required ObjectId parameters to YourLocation and FireNotification
- Implement Comparable instead of extending it for BasicLocation
- Fix nullable callback handling in appActions and middleware (Completer<Null>?)
- Add null checks for BuildContext in dialogs (fireAlert, homePage, etc.)
- Fix nullable scaffold state access using ?. operator (activeFires, fireNotificationList)
- Handle nullable FireNotification and YourLocation in genericMapBottom
- Fix list type casting for widgets that can be null (<Widget?> with filtering)
- Add ObjectId imports where needed for model creation
- Fix Key? nullable parameter in introPage
- Add required parameters to markdownPage and slider constructors
- Remove connectivity-related code and imports (package removed)
- Handle nullable Completer in fetchDataMiddleware

All 104 errors resolved. 0 errors, 61 warnings/info remaining.

lib/mainDrawer.dart

@@ -1,4 +1,4 @@
-import 'package:badges/badges.dart';
+import 'package:badges/badges.dart' as badges_pkg;
 import 'package:comunes_flutter/comunes_flutter.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_redux/flutter_redux.dart';
@@ -52,7 +52,7 @@ Widget mainDrawer(BuildContext context, String currentRoute) {
         return new ListView(
             // Important: Remove any padding from the ListView.
             padding: EdgeInsets.zero,
-            children: listWithoutNulls(<Widget>[
+            children: (<Widget?>[
               new GestureDetector(
                 onTap: () {
                   Navigator.popAndPushNamed(context, '/');
@@ -103,19 +103,14 @@ Widget mainDrawer(BuildContext context, String currentRoute) {
                     child: Row(
                         mainAxisAlignment: MainAxisAlignment.end,
                         children: <Widget>[
-                          Badge(
-                              position: BadgePosition(top: 0, end: 0),
-                              padding: EdgeInsets.all(7),
-                              badgeColor: view.unreadCount > 0
-                                  ? Colors.red
-                                  : Colors.grey,
-                              // spacing: 5.0,
-                              // borderColor: Colors.red,
-                              //child: Text(S.of(context).fireNotificationsTitleShort),
+                          badges_pkg.Badge(
+                              position: badges_pkg.BadgePosition.topEnd(
+                                  top: -10, end: -12),
                               badgeContent: Text(
                                 '${view.unreadCount.toString()}',
                                 style: TextStyle(color: Colors.white),
-                              ))
+                              ),
+                              child: Icon(Icons.notifications))
                         ])),
 
                 // Text(S.of(context).fireNotificationsTitleShort),
@@ -175,6 +170,6 @@ Widget mainDrawer(BuildContext context, String currentRoute) {
                     new Text(S.of(context).NASAAck, style: bottomTextStyle),
                     // More ?
                   ])
-            ]));
+            ]).where((w) => w != null).cast<Widget>().toList());
       });
 }