Flutter: Null safety features are disabled for this library when using InterstitialAd? _interstitialAd;

I'm integrating Interstitial ads in my flutter project. But When I declare like this

 InterstitialAd? _interstitialAd; 

I am getting this error:

33:17: Error: Null safety features are disabled for this library. Try removing the package language version or setting the language version to 2.12 or higher.   InterstitialAd? _interstitialAd;                 ^ lib/admob_service.dart:64:20: Error: Null safety features are disabled for this library. Try removing the package language version or setting the language version to 2.12 or higher.     _interstitialAd!.fullScreenContentCallback = FullScreenContentCallback(                    ^ lib/admob_service.dart:79:20: Error: Null safety features are disabled for this library. Try removing the package language version or setting the language version to 2.12 or higher.     _interstitialAd!.show(); 

I saw some stackoverflow answers and tried to upgrade the sdk version from

environment:   sdk: ">=2.7.0 <3.0.0" 

to

environment:   sdk: ">=2.12.0 <3.0.0" 

Then I got errors in my whole project.The error message is :

Error: Cannot run with sound null safety, because the following dependencies don't support null safety:  - package:local_database  - package:auto_size_text  - package:queue

Is there any way other than declaring this : InterstitialAd? _interstitialAd;