Avoid some crashes if router backstack is empty for whatever reason

app/src/main/java/eu/kanade/tachiyomi/ui/browse/BrowseController.kt

@@ -96,7 +96,7 @@ class BrowseController :
         /* It's possible to switch to the Library controller by the time setExtensionUpdateBadge
         is called, resulting in a badge being put on the category tabs (if enabled).
         This check prevents that from happening */
-        if (router.backstack.last().controller !is BrowseController) return
+        if (router.backstack.lastOrNull()?.controller !is BrowseController) return
 
         (activity as? MainActivity)?.binding?.tabs?.apply {
             val updates = preferences.extensionUpdatesCount().get()

app/src/main/java/eu/kanade/tachiyomi/ui/browse/migration/search/SearchController.kt

@@ -76,7 +76,7 @@ class SearchController(
             router.popController(this)
             if (newManga != null) {
                 val newMangaController = RouterTransaction.with(MangaController(newManga))
-                if (router.backstack.last().controller is MangaController) {
+                if (router.backstack.lastOrNull()?.controller is MangaController) {
                     // Replace old MangaController
                     router.replaceTopController(newMangaController)
                 } else {