The argument type 'dynamic Function(AppLovinAdListener)' can't be assigned to the parameter type 'dynamic Function(AppLovinAdListener?)'

Question

I'm trying to add Applovin Interstitial ads to flutter app. But it keep showing errors. my sdk is sdk:'>=2.12.0 <3.0.0'.

This are the errors displayed

A value of type 'bool?' can't be assigned to a variable of type 'bool'. Try changing the type of the variable, or casting the right-hand type to 'bool'.

The argument type 'dynamic Function(AppLovinAdListener)' can't be assigned to the parameter type 'dynamic Function(AppLovinAdListener?)'.

This is the sample code:

  const CollectionCard();

  @override
  State<CollectionCard> createState() => _CollectionCardState();
}

class _CollectionCardState extends State<CollectionCard> {
  AppLovinListener? get listener => null;

  void initState() {
    FlutterApplovinMax.initInterstitialAd('91b26a7777e1b455');
    super.initState();
  }

  bool isInterstitialVideoAvailable = false;

  @override
  Widget build(BuildContext context) {
    var size = MediaQuery.of(context).size;

    /*24 is for notifications bar on Android */
    final double itemHeight = (size.height - kToolbarHeight - 28) / 2;
    final double itemWidth = size.width / 4;
    return Container(
        child: Padding(
      padding: const EdgeInsets.fromLTRB(10.0, 0.0, 10.0, 10.0),
      child: Column(
        children: <Widget>[
          GridView.count(
            primary: true,
            padding: const EdgeInsets.fromLTRB(20, 0, 20, 20),
            crossAxisSpacing: 10, //Reduce Horizontal Spacing
            mainAxisSpacing: 10, //Reduce Vertical Spacing
            crossAxisCount: 3,
            physics: ScrollPhysics(),
            childAspectRatio: (6 / 8),
            // (itemWidth / itemHeight),
            shrinkWrap: true,
            children: <Widget>[
              Card(
                shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(15.0),
                ),
                elevation: 2,
                color: Theme.of(context).scaffoldBackgroundColor,
                child: InkWell(
                  onTap: () async {
                    isInterstitialVideoAvailable =
                        await FlutterApplovinMax.isInterstitialLoaded (listener);
                        
                    if (isInterstitialVideoAvailable) {
                      FlutterApplovinMax.showInterstitialVideo(
                          (AppLovinAdListener event) => listener (event));
                    }

                    Navigator.push(
                      context,
                      MaterialPageRoute(
                        builder: (ctx) => LearnPage(),
                      ),
                    );
                  },
                  child: Center(
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.spaceAround,
                      children: [
                        ImageIcon(
                          AssetImage('assets/icons/learn.png'),
                          color: kLightPrimary,
                          size: 60,
                        ), // Icon(
                        //   layout.icon,
                        //   size: 40,
                        // ),
                        Text(
                          'Learn',
                          style: TextStyle(
                            fontSize: 14,
                            fontWeight: FontWeight.bold,
                          ),
                        ),
                      ],
                    ),
                  ),
                ),
              ),

Below is an image of the code error

Answer 1

bool is a strict boolean - it can be true or false, nothing else.
bool? is a loose boolean or also called nullable - it can be true, false or null.
Same goes for the function.
The Plugin 'AppLovin' I assume you are using or the package, or the function you declared, doesn't support null-safety, which means that it's functions can return null.
In order to solve the issue, you need to make the types nullable by adding a question mark after the type declaration (i.e. bool a; will become - bool? a; and make sure you don't call a method on a null. Since your variables and methods can return null, the editor will inform you before hand that this (the variable) might be null and calling a method on it will throw a MethodNotFoundException. Therefore, you should add your own logic to assure that this doesn't happen and satisfy the compiler by adding null-checks (exclamation marks - '!') to method calls which works as a form of asserting the compiler that this value isn't null, despite it being nullable. If it so happens that you do call a null on a value with a null check, at run time (after you press the run and load your page) you will get the following exception - Null-check used on a null value.