Null check operator used on a null value for flutter

So I am switching a project to flutter null safety but i am getting Null check operator used on a null value error i tried looking into other similar issue tried but its not working for me the complete error is as following

The following _CastError was thrown building HomeBanner(dirty, dependencies: [MediaQuery, _InheritedProviderScope<HomeViewModel>], state: _HomeBannerState#2de56):
Null check operator used on a null value

The relevant error-causing widget was: 
HomeBanner file:///D:/fabricoMAt/lib/ui/views/home_screen.dart:26:17
When the exception was thrown, this was the stack: 
#0      HomeViewModel.initData (package:knoty/business_logic/view_models/home_viewmodel.dart:25:33)
#1      _HomeBannerState._getImageSliders (package:knoty/ui/widgets/home_banner.dart:30:18)
#2      _HomeBannerState.build (package:knoty/ui/widgets/home_banner.dart:20:23)
#3      StatefulElement.build (package:flutter/src/widgets/framework.dart:4612:27)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4495:15)
...
====================================================================================================

======== Exception caught by widgets library =======================================================
Null check operator used on a null value
The relevant error-causing widget was: 
WhomeServices file:///D:/fabricoMAt/lib/ui/views/home_screen.dart:43:17

And so onn this is coming at multiple place and i am not getting any idea how to fix this.

  HomeViewModel model = Provider.of<HomeViewModel>(context);
Size screenSize = MediaQuery.of(context).size;
return model.initData == null   //home_banner line 30
    ? [1, 2, 3, 4]
        .map(
          (item) => Shimmer.fromColors(
            child: Container(
              margin: EdgeInsets.only(right: 8),
              width: screenSize.width * .7,
              height: screenSize.width * .35,
              decoration: BoxDecoration(
                borderRadius: BorderRadius.all(
                  Radius.circular(8.0),
                ),
                color: Colors.black,
              ),
            ),
            baseColor: Colors.grey.shade300,
            highlightColor: Colors.grey.shade100,
          ),
        )
        .toList()

the other mentioned line

 Init? _initData;

 City get selectedCity {
  return _initData!.cities
    .firstWhere((element) => element.id == _initData!.selectedCityId);
 }

 Init get initData => _initData!;// home_viewmodel.dart 25

Answer

In home_banner at line 30, you are using initData method which returns a not nullable value.

So make a nullable return type of initData and remove ! from _initData.

Init? get initData => _initData;// home_viewmodel.dart 25

And if you are using get selectedCity in your code without checking the null value of _initData, this may cause an error in your code, So add a null check in this method also.

City get selectedCity {
   City? city = _initData?.cities
  .firstWhere((element) => element.id == _initData?.selectedCityId);

  if(city==null){
     return City();// return default city
  }else{
     return city;
  }
}