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/fireAlert.dart

@@ -65,9 +65,11 @@ class _FireAlertState extends State<FireAlert> {
                 .of(context)
                 .tweetAboutSelf(yourLocation.description, '#IF$where'));
           }).catchError((onError) {
-            ScaffoldMessenger.of(context).showSnackBar(new SnackBar(
-                content: new Text(
-                    S.of(_scaffoldKey.currentContext).errorFirePlaceDialog)));
+            final ctx = _scaffoldKey.currentContext;
+            if (ctx != null) {
+              ScaffoldMessenger.of(ctx).showSnackBar(new SnackBar(
+                  content: new Text(S.of(ctx).errorFirePlaceDialog)));
+            }
           });
         },
       ),