Dart Flutter _CastError (Null check operator used on a null value) listyukle Error

I have a code like this:

  void saveList() async {
    final prefences = await SharedPreferences.getInstance();
    prefences.setStringList("tests", prefences.getStringList("tests")! + ["English / A1 / Family / Test 1"]);
    setState(() {
    });
  }

I’m calling the saveList function somewhere. When I call it, I get an error like this:

_CastError (Null check operator used on a null value) saveList

It gives the error on this line:

prefences.setStringList("tests", prefences.getStringList("tests")! + ["Ingilizce / A1 / Aile / Test 1"]);

How can I solve it? Thank you for your help.

>Solution :

I think the first time you call this line, "tests" has never been set before in prefences, then prefences.getStringList("tests") is null.

Try this to fix it:

  void saveList() async {
    final prefences = await SharedPreferences.getInstance();
    List<String> previousList = prefences.getStringList("tests") ?? [];
    prefences.setStringList("tests", previousList + ["English / A1 / Family / Test 1"]);
    setState(() {
    });
  }