Why does the Null check operator used on a null value trigger here?

Question

Hey I have trouble with the Null check operator used on a null value error. In general I nearly have no problems with this, just in one case.

So I have a site where you can chose between two options & this works always, but in this one case not

Because of that I thought that I have flaws in my Dataset, so that a value just isnt defined. I checked & it seemed correct:

Here I coppied in a part of the flawed data:

                       {
                      "title":"105 PS",
                      "unterscheidungsData":[
                         {
                            "difference":"Hubraum 1422 cm³",
                            "title":"1.4 TDI DPF BlueMotion Technology",
                            "wholeText ":"Ölverlust\n\nBremse\n\nBatterie(2009-2013)\n\nSteuerkette(2009 - 2011)\n\nZündspule(2009 - 2010)\n\nZündkerze(2009 - 2013)\n\nScheinwerfer(2010)"
                         },
                         {
                            "difference ":"Hubraum 1598 cm³",
                            "title ":"1.6 TDI(DPF)",
                            "wholeText ":"Ölverlust\n\nBremse\n\nBatterie(2009-2013)\n\nSteuerkette(2009 - 2011)\n\nZündspule(2009 - 2010)\n\nZündkerze(2009 - 2013)\n\nScheinwerfer(2010)"
                         }
                      ]
                   }

& here is data that works perfectly fine, I compared them & didnt really notice any differences in the syntax

                      {
                      "title":"150 PS",
                      "unterscheidungsData":[
                         {
                            "difference":"Sportwagen, Crosswagon",
                            "title":"1.9 16V JTD M-Jet",
                            "wholeText":"abbrechende Drallklappen können zu Turbo- oder Motorschaden führen\n\nDefekte Ölpumpen\n\nDefekte Lichtmaschinen\n\nverschlissene Zweimassenschwungräder\n\nVerrußte und hängende Abgasrückführungsventile\n\nAusfall Bremsservo\n\nUndichte Wasserkühler auf der Innenseite\n\ngerissene Keilrippenriemen\n\ndefekte Anlasser"
                         },
                         {
                            "difference":"Normal",
                            "title":"2.4 10V JTD",
                            "wholeText":"Verrußte und hängende Abgasrückführungsventile\n\nAusfall Bremsservo\n\nUndichte Wasserkühler auf der Innenseite\n\ngerissene Keilrippenriemen\n\ndefekte Anlasser\n\nBeschädigungen der Ölwanne"
                         }
                      ]
                   },

                           

So lastly I thought that my code has some flaws (this is my listview builder):

ListView.builder(
          itemCount: this.leistungData!.unterscheidungsData!.length,
          itemBuilder: (context, index) => GestureDetector(
            onTap: () {
              Navigator.push(context,
                  MaterialPageRoute(builder: (context) => AutoProbleme(
                    variantData: variantData,
                    unterscheidungsData: leistungData!.unterscheidungsData![index],
                    modell: modell,
                    modelData: modelData,
                  )));
            },
            child: Container(
              margin: EdgeInsets.all(10),
              height: MediaQuery.of(context).size.height * 0.05,
              width: MediaQuery.of(context).size.width * 0.9,
              child: Align(
                alignment: Alignment.center,
                child: Text(
                  this.leistungData!.unterscheidungsData![index].difference!,
                  textScaleFactor: 1.4,
                ),
              ),
              decoration: BoxDecoration(
                  color: Colors.white,
                  borderRadius: BorderRadius.circular(16)),
            ),
          )),

& I have to say that I didnt find anything that would justify this behaviour. For me the data looks fine & it works nearly always, but nevertheless I get this error. Do you have any ideas what the problem could be?

Answer 1

You should make sure that all expressions with a nullable type aren't null if you are using the null assertion operator !.

The null assertion operator (!)

If you’re sure that an expression with a nullable type isn’t null, you can use a null assertion operator (!) to make Dart treat it as non-nullable. By adding ! just after the expression, you tell Dart that the value won’t be null, and that it’s safe to assign it to a non-nullable variable.

Pay attention to the following lines from the docs:

If you’re wrong, Dart throws an exception at run-time. This makes the ! operator unsafe, so don’t use it unless you’re very sure that the expression isn’t null.

bad ❌

int? number;

print(number! > 0);

good ✅

int? number;

if(number != null) {
   print(number > 0);
}

I would recommend that you check whether leistungData is null and show a loading state using CircularProgressIndicator in case it is null:

@override
Widget build(BuildContext context) {
  if (leistungData == null || leistungData?.unterscheidungsData == null) {
    return CircularProgressIndicator();
  }

  return ListView.builder(
    itemCount: leistungData!.unterscheidungsData!.length,
    itemBuilder: (context, index) => GestureDetector(
      onTap: () {
        Navigator.push(
          context,
          MaterialPageRoute(
            builder: (context) => AutoProbleme(
              variantData: variantData,
              unterscheidungsData: leistungData!.unterscheidungsData![index],
              modell: modell,
              modelData: modelData,
            ),
          ),
        );
      },
      child: Container(
        margin: EdgeInsets.all(10),
        height: MediaQuery.of(context).size.height * 0.05,
        width: MediaQuery.of(context).size.width * 0.9,
        child: Align(
          alignment: Alignment.center,
          child: Text(
            leistungData!.unterscheidungsData![index]?.difference ?? '',
            textScaleFactor: 1.4,
          ),
        ),
        decoration: BoxDecoration(
          color: Colors.white,
          borderRadius: BorderRadius.circular(16),
        ),
      ),
    ),
  );
}

Answer 2

This error comes when non-null operator is used on a null value

int? data;


void main() {
  print(data!.value); 
}

The above code will give an error if the data value is not assigned.

Solution:

?. operator used to check null values, and ?? is used to provide default values if the given value is null

 int? data;
    
    
    void main() {
      print(data?.value ?? "Default Value"); 
    }