fix another crash in ViewImageFragment

app/src/main/java/com/keylesspalace/tusky/fragment/ViewImageFragment.kt

@@ -105,9 +105,11 @@ class ViewImageFragment : ViewMediaFragment() {
                         override fun onError() {
                             // if there's no image in cache, load from network and start transition
                             // immediately.
+                            if (isAdded) {
                                 photoActionsListener.onBringUp()
                                 loadImageFromNetwork(url, photoView)
                             }
+                        }
                     })
         } else {
             // if we're not initial page, don't bother.