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/fireNotificationList.dart

@@ -67,10 +67,9 @@ class _FireNotificationListState extends State<FireNotificationList> {
     String prefix = "";
 
     // FIXME (this can fails if you don't have a location for this notif, for instance during tests)
-    YourLocation yl =
-        yourLocations.singleWhere((yl) => yl.id == notif.subsId);
+    YourLocation yl = yourLocations.singleWhere((yl) => yl.id == notif.subsId);
     prefix = '${yl.description}. ';
-  
+
     return new ListTile(
         dense: true,
         leading: const Icon(Icons.whatshot),
@@ -190,16 +189,16 @@ class _FireNotificationListState extends State<FireNotificationList> {
                   leading: IconButton(
                     icon: Icon(Icons.menu),
                     onPressed: () {
-                      _scaffoldKey.currentState.openDrawer();
+                      _scaffoldKey.currentState?.openDrawer();
                     },
                   ),
-                  actions: listWithoutNulls(<Widget>[
+                  actions: (<Widget?>[
                     hasFireNotifications
                         ? IconButton(
                             icon: Icon(Icons.delete),
                             onPressed: () => _showConfirmDialog(view))
                         : null
-                  ])),
+                  ]).where((w) => w != null).cast<Widget>().toList()),
               body: !view.isLoaded
                   ? new FiresSpinner()
                   : !hasFireNotifications
@@ -253,7 +252,7 @@ class _FireNotificationListState extends State<FireNotificationList> {
             ),
           ),
           actions: <Widget>[
-            new FlatButton(
+            new TextButton(
               child: new Text(S.of(context).DELETE),
               onPressed: () {
                 view.onDeleteAll();