Null check operator used on a null value

[robin2005]

======== Exception caught by foundation library ====================================================
The following _CastError was thrown while dispatching notifications for SwiperController:
Null check operator used on a null value

When the exception was thrown, this was the stack:
#0 ScrollPosition.viewportDimension (package:flutter/src/widgets/scroll_position.dart:172:53)
#1 _PagePosition.getPixelsFromPage (package:flutter/src/widgets/page_view.dart:382:19)
#2 PageController.animateToPage (package:flutter/src/widgets/page_view.dart:197:16)
#3 _TransformerPageViewState.onChangeNotifier (package:flutter_swiper_null_safety/src/transformer_page_view/transformer_page_view.dart:590:12)
#4 ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:243:25)
...
The SwiperController sending notification was: Instance of 'SwiperController'

Flutter 2.2.1版本下:

每滑动一页就报一次异常

此问题怎么解决呢？