Null check operator used on a null value

[sunfa-airgram]

flutter: ══╡ EXCEPTION CAUGHT BY FOUNDATION LIBRARY ╞════════════════════════════════════════════════════════
flutter: The following _CastError was thrown while dispatching notifications for SwiperController:
flutter: Null check operator used on a null value
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0 ScrollPosition.viewportDimension (package:flutter/src/widgets/scroll_position.dart:172:53)
flutter: #1 _PagePosition.getPixelsFromPage (package:flutter/src/widgets/page_view.dart:382:19)
flutter: #2 PageController.animateToPage (package:flutter/src/widgets/page_view.dart:197:16)
flutter: #3 _TransformerPageViewState.onChangeNotifier (package:flutter_swiper_null_safety/src/transformer_page_view/transformer_page_view.dart:590:12)
flutter: #4 ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:243:25)
flutter: #5 IndexController.next (package:flutter_swiper_null_safety/src/transformer_page_view/index_controller.dart:30:5)
flutter: #6 _SwiperTimerMixin._onTimer (package:flutter_swiper_null_safety/src/swiper.dart:369:18)
flutter: (elided 10 frames from class _RawReceivePortImpl, class _Timer, and dart:async)
flutter:
flutter: The SwiperController sending notification was:
flutter: Instance of 'SwiperController'