Dart closure return type problem after migrating to null safety

Question

The below code used to work before migrating to Flutter 2 and null safety. I just can't figure out what the correct return type should be. The error I get is:

The return type 'List<PopupMenuItem<dynamic>?>' isn't a 'List<PopupMenuEntry<dynamic>>', as required by the closure's context.

And the code is:

 Widget _parentMenuSelector() {
    return PopupMenuButton<dynamic>(
      child: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            Icon(Icons.content_copy),
            Text('Copy to another menu'),
          ],
        ),
      ),
      onSelected: (dynamic selectedMenu) {
        Map<String, dynamic> selMenu = selectedMenu;
        print(selMenu['id']);
        model!.copyMenuItem(selMenu['id']);
      },
      itemBuilder: (BuildContext context) {
        return model!.restaurant!.restaurantMenus!.values.map((dynamic item) { //ERROR
          Map<String, dynamic> itemMap = item;                                 //ERROR
          if (itemMap['name'] != model!.menu!.name) {                          //ERROR
            return PopupMenuItem<dynamic>(                                     //ERROR
              child: Text('Copy to ${itemMap['name']}'),                       //ERROR
              value: item,                                                     //ERROR
            );                                                                 //ERROR
          }                                                                    //ERROR
        }).toList();                                                           //ERROR
      },
    );
  }

Solution

As suggested by jamesdlin, here's what worked for me at least syntactically for now. I'm not sure yet if it's going to work functionally - I haven't build the app yet as I'm still fixing null-safety migration issues.

  Widget _parentMenuSelector() {
    return PopupMenuButton<dynamic>(
      child: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            Icon(Icons.content_copy),
            Text('Copy to another menu'),
          ],
        ),
      ),
      onSelected: (dynamic selectedMenu) {
        Map<String, dynamic> selMenu = selectedMenu;
        print(selMenu['id']);
        model!.copyMenuItem(selMenu['id']);
      },
      itemBuilder: (BuildContext context) {
        return model!.restaurant!.restaurantMenus!.values.map((dynamic item) {
          Map<String, dynamic> itemMap = item;
          if (itemMap['name'] != model!.menu!.name) {
            return PopupMenuItem<dynamic>(
              child: Text('Copy to ${itemMap['name']}'),
              value: item,
            );
          } else {
            return PopupMenuItem<dynamic>(
              child: null,
            );
          }
        }).toList();
      },
    );
  }

Comments

[Benyamin]

Here remove the type and see what happens Map<String, dynamic> itemMap = item....; =>itemMap = .... where have u declared PopupMenuEntry?

[jamesdlin]

Your callback function contains an if statement, but it doesn't return a value along all code paths. It therefore implicitly returns null if the if-condition isn't met, causing your callback function to return a PopupMenuItem<dynamic>? instead of a PopupMenuItem<dynamic> as expected. Fix your code to return a PopupMenuItem along all code paths, or use .where or .whereType if you want to filter the list.

[Ernesto]

@jamesdlin - Thanks for the advice. It worked! Now I just need to figure out what exactly I need to return for that specific path.