Fix IndexOutOfBoundsException in onPause (#3581)

Use `getOrNull` instead of `get`, which was occasionally throwing IndexOutOfBoundsException.

app/src/main/java/com/keylesspalace/tusky/components/notifications/NotificationsFragment.kt

@@ -453,7 +453,7 @@ class NotificationsFragment :
         // Save the ID of the first notification visible in the list
         val position = layoutManager.findFirstVisibleItemPosition()
         if (position >= 0) {
-            adapter.snapshot()[position]?.id?.let { id ->
+            adapter.snapshot().getOrNull(position)?.id?.let { id ->
                 viewModel.accept(InfallibleUiAction.SaveVisibleId(visibleId = id))
             }
         }