Check view is non-null before scrolling, fix crash in NotificationsFragment (#3594)

The posted message runs at the end of the message queue, by which time the view may no longer exist.

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

@@ -211,10 +211,12 @@ class NotificationsFragment :
             override fun onItemRangeInserted(positionStart: Int, itemCount: Int) {
                 if (positionStart == 0 && adapter.itemCount != itemCount) {
                     binding.recyclerView.post {
+                        if (getView() != null) {
                             binding.recyclerView.scrollBy(0, Utils.dpToPx(requireContext(), -30))
                         }
                     }
                 }
+            }
         })
 
         // update post timestamps