future local variable doesn't save the value and returns null

I've assigned a value for a local variable inside a function (Future), but the result always gives a null value. Is there any wrong code in my code?

enum StatusUser { telahMasuk, belumMasuk, berhasilMasuk, gagalMasuk }

class MyClass {
  Future<StatusUser> muatSeting(String? key) async {
    StatusUser? sp;

    final data = await getUserData();
    try {
      if (data != null) {
        final dataToken = jsonDecode(
            APIUser().decodeJWT(data['token'].toString().split('.')[1]));
        if (new DateTime.now().millisecondsSinceEpoch -
                dataToken['tgl_verify'] <
            864000) {
          sp = StatusUser.telahMasuk;
        }
      } else {
        sp = StatusUser.belumMasuk;
      }
    } catch (e) {
      print(e);
    }
    return sp!;
  }
}

When I debugging, its shows _CastError (Null check operator used on a null value). Whereas at previous version of Flutter, it works. I want that this future returning an enum value from variable sp.

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The null check operator is failing here because sp is null.

return sp!;

Looking from the if statement, I think this condition fails (new DateTime.now().millisecondsSinceEpoch - dataToken['tgl_verify'] < 864000) and therefore sp is never set.

if (data != null) {
    final dataToken = jsonDecode(
        APIUser().decodeJWT(data['token'].toString().split('.')[1]));
    if (new DateTime.now().millisecondsSinceEpoch -
            dataToken['tgl_verify'] <
        864000) { // this if statement never runs so sp is null.
      sp = StatusUser.telahMasuk;
    }
  }

If you want StatusUser.belumMasuk to be returned by default, then you either should initialize sp to use that as its default value:

Future<StatusUser> muatSeting(String? key) async {
  StatusUser sp = StatusUser.belumMasuk;

  final data = await getUserData();
  try {
    if (data != null) {
      final dataToken = jsonDecode(
          APIUser().decodeJWT(data['token'].toString().split('.')[1]));
      if (DateTime.now().millisecondsSinceEpoch - dataToken['tgl_verify'] <
          864000) {
        sp = StatusUser.telahMasuk;
      }
    }
  } catch (e) {
    print(e);
  }
  return sp;
}

or use the ?? operator:

Future<StatusUser> muatSeting(String? key) async {
  StatusUser? sp;

  final data = await getUserData();
  try {
    if (data != null) {
      final dataToken = jsonDecode(
          APIUser().decodeJWT(data['token'].toString().split('.')[1]));
      if (DateTime.now().millisecondsSinceEpoch - dataToken['tgl_verify'] <
          864000) {
        sp = StatusUser.telahMasuk;
      }
    }
  } catch (e) {
    print(e);
  }
  return sp ?? StatusUser.belumMasuk;
}

Both cases would allow you to remove the else blocks and additionally would handle the case where an exception is thrown within the try block. (Incidentally, you should avoid catches without on clauses.)

In your answer, you made a remark:

> I'm in confuse with 'why nested if (child) when without else and false condition dosen't returning value that was set on else from parent if'

Why would it? That's not how if-else works. The outer else block corresponds only to the outer if condition.

Imagine:

if (condition1) {
  if (condition2) {
    doStuff();
  }

  doLotsOfOtherStuff();
} else {
  handleCondition1Failed();
}

If condition2 is false, do you expect doLotsOfOtherStuff() to be skipped? But if so, then doLotsOfOtherStuff() should be moved to be within the if (condition2) block. Do you expect doLotsOfOtherStuff() and handleCondition1Failed() to both be executed? Either of those choices would be unintuitive and very hard to follow for most people.

If you want the else block to be executed if either condition1 or condition2 fails, then you should write it as:

var succeeded = false;

if (condition1) {
  if (condition2) {
    doStuff();
    succeeded = true;
  }

  doLotsOfOtherStuff();
}

if (!succeeded) {
  handleFailure();
}

I've solved it with adding else on nested if

if (data != null) {
    final dataToken = jsonDecode(
        APIUser().decodeJWT(data['token'].toString().split('.')[1]));
    if (new DateTime.now().millisecondsSinceEpoch -
            dataToken['tgl_verify'] < 864000) {
        sp = StatusUser.telahMasuk;
    } else {
        sp = StatusUser.belumMasuk;
    }
  } else {
    sp = StatusUser.belumMasuk;
  }

I'm in confuse with 'why nested if (child) when without else and false condition dosen't returning value that was set on else from parent if'