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

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.

CodePudding user response：

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(() {
    });
  }

CodePudding user response：

I believe your getStringList where it might be null even though you have used !. According to the documentation

// Try reading data from the 'items' key. If it doesn't exist, returns null.
final List<String>? items = prefs.getStringList('items');

Thus, you can change your code in order to make sure the value is never null but it's an array even though it will be empty. Your example will be


  void saveList() async {
    final prefences = await SharedPreferences.getInstance();

    final existingValues =
        prefences.getStringList("tests") ?? []; // always a List

    prefences.setStringList(
      "tests",
      [...existingValues, "English / A1 / Family / Test 1"],
    );

    setState(() {});
  }