The return type 'Null' isn't a 'University', as required by the closure's context

I'm stuck trying to test my code. but I get the error that "The return type 'Null' isn't a 'University', as required by the closure's context.

Any suggestions on how to fix this?

Here's the code:

static Future<void> deleteFaculty(String universityCode, String facultyCode) async {
        University? targetUniversity = _universities.firstWhere(
              (uni) => uni.code == universityCode,
          orElse: () => null, // doesn't work.
        );

        if (targetUniversity != null) {
          int initialLength = targetUniversity.faculties.length;
          targetUniversity.faculties.removeWhere((f) => f.code == facultyCode);
          int finalLength = targetUniversity.faculties.length;

          if (finalLength < initialLength) {
            await saveUniversities(_universities); // Await saving universities after modification
          } else {
            throw Exception('Faculty $facultyCode not found in university ${targetUniversity.name}');
          }
        } else {
          throw Exception('University $universityCode not found');
        }
      }`

I will appreciate any solutions guys

Assuming _universities is of type Iterable<University>, you can use the firstWhereOrNull extension method instead.

Alternatively, you can do something like this:

University? targetUniversity = _universities.cast<University?>().firstWhere(
      (uni) => uni!.code == universityCode,
  orElse: () => null, 
);