Fix crash when getting account relationship fails

UIDeferredMenuElement completion handler should only be called from the
main thread

Tusker/Screens/Utilities/Previewing.swift

@@ -67,7 +67,9 @@ extension MenuPreviewProvider {
                     guard let self = self,
                           case let .success(results, _) = response,
                           let relationship = results.first else {
-                              elementHandler([])
+                              DispatchQueue.main.async {
+                                  elementHandler([])
+                              }
                               return
                           }
                     let following = relationship.following