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Android Studio – I am getting null parameter error in Flutter, how can I solve it?

I’m learning Flutter and I’m making an app as a demo, but I couldn’t solve this error. Even though I have given values ​​to the parameter, when I run it, I get an error that it cannot be null.

student.dart code

class Student {
  String firstName;
  String lastName;
  String status;
  int grade;

  Student(String firstName, String lastName, int grade) {
    this.firstName = firstName;
    this.lastName = lastName;
    this.status = "Geçti";
    this.grade = grade;
  }
}

main.dart code

List<Student> students = [Student("Engin", "Demiroğ", 75), Student("İsmail", "Güner", 65), Student("Aykut", "Elmas", 88), Student("Cengiz", "Han", 100)];

@override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Demo1"),
      ),
      body: buildBody(context),
    );
  }

  Widget buildBody(BuildContext context) {
    return Column(
      children: [
        Expanded(
          child: ListView.builder(
              itemCount: students.length,
              itemBuilder: (BuildContext context, int index) {
                return Text(students[index].firstName);
              }),
        ),

The error I got

enter image description here

Tags:

6

Answers


  1. 0

    Add the question mark with the type of each property to indicate that it is nullable:

    class Student {
  String? firstName;
  String? lastName;
  String? status;
  int? grade;

    Dart is null-safe, so since you’re declaring new properties of a class without the question mark, it will interpret that the value will never be null, but initially their values are all null, thus the error. I suggest to read more on the null safety feature since it’s a pretty important concept for Dart/ Flutter.

    P/s: To use the nullable property (for example String? firstName, use it with ?? for those widgets that don’t take in nullable value. For example:

    Text(student.firstName ?? ‘Empty name’)

    Here, the ?? has meaning that, if student.firstName is null, use this Empty name string as value. Since Text requires a non-null value, we need to make sure that in case the firstName is null, we have a default value in place to avoid passing nullable value in.

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  2. 0

    Dart has recently introduced null-safety. It prevents you from making a lot of mistakes. For more info: https://dart.dev/null-safety/understanding-null-safety

    The best way to solve your problem is to understand what variables can be nullable or non-nullable.

    class Student {
  String? firstName;
  String? lastName; ///nullable
  String status; /// non-nullable
  int? grade;

  Student({this.firstName, this.lastName, this.status = 'Geçti', this.grade});
}

    In this case, only the variable status can’t be null, and it’s value is set to "Geçti" by the constructor. All the other variables can be null.

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  3. 0

    Flutter can be serious concerning null values and initialization. Even if practically you assign values, it does not ‘believe’ you, and throws a build error. See here for example. But for as a quick fix you can mark the members of Student class as late, practically you promise to Flutter, that later you will assign values:

    class Student {
  late String firstName;
  late String lastName;
  late String status;
  late int grade;
...

    You have many other options, but this one works as a starter, I tried it.

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  4. 0

    From Dart 2.12 you need to specify if your parameters are nullable or vice versa. Please see below code

    class Student {
  final String firstName;
  final String lastName;
  final String status;
  final int grade;

  Student({required this.firstName, required this.lastName, required this.status, required this.grade,});
}
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  5. 0

    If you enable null safety, then you must initialize the values of non-nullable variables before you use them. Source: documentation for more info.

    Null safety is enabled by default. As stated there, you need to assign a default value for those variables.

    Your class can be rewritten using dart syntax sugar for initializing variables (documentation)

    class Student {
  String firstName = "";
  String lastName = "";
  String status = "";
  int grade = 0;
    
  Student(this.firstName, this.lastName, this.grade);
}

    Using that syntax for initializing variables we don’t need to specify a default value for them, so removing the unnecesary default values:

    class Student {
  String firstName;
  String lastName;
  int grade;
  String status = "";
    
  Student(this.firstName, this.lastName, this.grade);
}

    It’s also common to declare variables initialized in the constructor as final (can only be initialized once)

    class Student {
  final String firstName;
  final String lastName;
  final int grade;
  String status = "";
    
  Student(this.firstName, this.lastName, this.grade);
}

    I recommend taking a look at the Dart language tour

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  6. 0

    This happens because the type of your variables are not nullable but you didn’t initialize them with a value. To fix this use the keyword late.

    class Student {
  late String firstName;
  late String lastName;
  late String status;
  late int grade;

  Student(this.firstName, this.lastName, this.grade) {
    this.status = "Geçti";
  }
}
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