Null pointer exception in Showcase widget - flutter

Question

I have used showcaseview: ^1.1.1
Gives this error. :
Null check operator used on a null value
I/flutter (14014): #0 _OverlayBuilderState.addToOverlay
package:showcaseview/src/layout_overlays.dart:163
I/flutter (14014): #1 _OverlayBuilderState.showOverlay
package:showcaseview/src/layout_overlays.dart:155
I/flutter (14014): #2 _OverlayBuilderState.initState.<anonymous closure>
package:showcaseview/src/layout_overlays.dart:120
I/flutter (14014): #3 SchedulerBinding._invokeFrameCallback
package:flutter/…/scheduler/binding.dart:1144
I/flutter (14014): #4 SchedulerBinding.handleDrawFrame
package:flutter/…/scheduler/binding.dart:1090
I/flutter (14014): #5 SchedulerBinding._handleDrawFrame
package:flutter/…/scheduler/binding.dart:998
I/flutter (14014): #6 _rootRun (dart:async/zone.dart:1354:13)
I/flutter (14014): #7 _CustomZone.run (dart:async/zone.dart:1258:19)
I/flutter (14014): #8 _CustomZone.runGuarded (dart:async/zone.dart:1162:7)
I have Passed the ShowCaseWidget like this
ShowCaseWidget(builder:Builder((context) => MaterialApp.router(...),
),
);
And then in the Page I want to show the showcase,
#override
void initState() {
super.initState();
WidgetsBinding.instance!.addPostFrameCallback(
(_) => ShowCaseWidget.of(context)!.startShowCase([_one]));
}
And in the widget tree
Showcase(
key: _one,
description: "This is test descriptiton",
child: IconButton(
icon: Image.asset('assets/images/invite.png'),
onPressed: () {
// getIt<IAnalyticsService>().logCreateNavigateClick();
// showModalBottomSheet(
// context: context,
// builder: (context) => const CreateSheet(),
// );
AutoRouter.of(context).push(SelectTopicRoute());
},
),
),