Optimize mobile - Avoid creating unnecessary widgets (#268)

* Avoid creating unnecessary widgets

* more flexible null handling and runtime errors prevention

mobile/lib/modules/backup/views/backup_album_selection_page.dart

@@ -188,11 +188,7 @@ class BackupAlbumSelectionPage extends HookConsumerWidget {
                           color: Colors.grey[700]),
                     ),
                     trailing: Text(
-                      ref
-                          .watch(backupProvider)
-                          .allUniqueAssets
-                          .length
-                          .toString(),
+                      '${ref.watch(backupProvider).allUniqueAssets.length}',
                       style: const TextStyle(fontWeight: FontWeight.bold),
                     ),
                   ),
@@ -203,7 +199,7 @@ class BackupAlbumSelectionPage extends HookConsumerWidget {
 
           ListTile(
             title: Text(
-              "Albums on device (${availableAlbums.length.toString()})",
+              "Albums on device (${availableAlbums.length})",
               style: const TextStyle(fontWeight: FontWeight.bold, fontSize: 14),
             ),
             subtitle: Padding(

mobile/lib/modules/backup/views/backup_controller_page.dart

@@ -96,12 +96,11 @@ class BackupControllerPage extends HookConsumerWidget {
           child: Column(
             crossAxisAlignment: CrossAxisAlignment.start,
             children: [
-              !isAutoBackup
-                  ? const Text(
-                      "Turn on backup to automatically upload new assets to the server.",
-                      style: TextStyle(fontSize: 14),
-                    )
-                  : Container(),
+              if (!isAutoBackup)
+                const Text(
+                  "Turn on backup to automatically upload new assets to the server.",
+                  style: TextStyle(fontSize: 14),
+                ),
               Padding(
                 padding: const EdgeInsets.only(top: 8.0),
                 child: OutlinedButton(
@@ -189,7 +188,7 @@ class BackupControllerPage extends HookConsumerWidget {
           ),
         );
       } else {
-        return Container();
+        return const SizedBox();
       }
     }