Null check operator used on a null value

[harithwick]

got this error after upgrading the app to NULL SAFETY. Issue is at chat_view.dart line 459

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

When the exception was thrown, this was the stack:
#0 DashChatState.widgetBuilt. (package:dash_chat/src/chat_view.dart:459:29)
#1 ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:308:24)
#2 ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:308:24)
#3 ScrollPosition.notifyListeners (package:flutter/src/widgets/scroll_position.dart:969:11)
#4 ScrollPosition.setPixels (package:flutter/src/widgets/scroll_position.dart:278:9)
#5 ScrollPositionWithSingleContext.setPixels (package:flutter/src/widgets/scroll_position_with_single_context.dart:82:18)
#6 BallisticScrollActivity.applyMoveTo (package:flutter/src/widgets/scroll_activity.dart:566:21)
#7 BallisticScrollActivity._tick (package:flutter/src/widgets/scroll_activity.dart:553:10)
#8 AnimationLocalListenersMixin.notifyListeners (package:flutter/src/animation/listener_helpers.dart:155:19)
#9 AnimationController._tick (package:flutter/src/animation/animation_controller.dart:830:5)
#10 Ticker._tick (package:flutter/src/scheduler/ticker.dart:238:12)
#11 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1144:15)
#12 SchedulerBinding.handleBeginFrame. (package:flutter/src/scheduler/binding.dart:1057:11)
#13 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:539:8)
#14 SchedulerBinding.handleBeginFrame (package:flutter/src/scheduler/binding.dart:1055:17)
#15 SchedulerBinding._handleBeginFrame (package:flutter/src/scheduler/binding.dart:974:5)
#19 _invoke1 (dart:ui/hooks.dart:170:10)
#20 PlatformDispatcher._beginFrame (dart:ui/platform_dispatcher.dart:286:5)
#21 _beginFrame (dart:ui/hooks.dart:104:31)
(elided 3 frames from dart:async)
The ScrollController sending notification was: ScrollController#dbf76(one client, offset 0.0)