Non-nullable instance field must be initialized

(Your code was fine before Dart 2.12, null safety)

With null safety, Dart doesn't know if you actually assigned a value to count. Dart can see initialization in three ways:

1. At the time of declaration:

int count = 0;

2. In the initializing formals parameters:

Foo(this.count);

3. In the initializer list:

Foo() : count = 0;

So, according to Dart, count was never initialized in your code and hence the error. The solution is to either initialize it in 3 ways shown above or just use the late keyword which will tell Dart that you are going to initialize the variable later.

4. Use the late keyword:

class Foo {
  late int count; // No error
  void bar() => count = 0;
}

5. Make variable nullable:

class Foo {
  int? count; // No error
  void bar() => count = 0;
}

These are new rules about Dart about null safety

class Note {
  late int _id;
  late String _title;
  late String? _description;
  late String _date;
  late int _priority;
}

make sure before your variable put late

Use the late keyword to initialize a variable when it is first read, rather than when it's created.

class Questionz {
    late String questionText;
    late bool questionAnswer;

    Questionz({required String t, required bool a}) {
        questionText = t;
        questionAnswer = a;
    }
}

IN my case i found giving ? and ! to the variable helpful:

 double? _bmi; // adding ? to the double 

  String calculateBMI(){

    _bmi=weight/pow(height/100, 2);
     return  _bmi!.toStringAsFixed(1);// adding ! to the private variable


}

 String getResult(){
    if(_bmi!>=25){ //adding ! to the private variable
     return 'Overweight';
     } else if (_bmi!>=18.5)
   {
      return 'normal';
    }else{return 'underweight';}

in pubspec.yaml if you are using : environment: sdk: ">=2.12.0 <3.0.0"

change to environment: sdk: ">=2.7.0 <3.0.0"

2.12.0 null safety feature is on & 2.7.0 null safety feature is off

tip : instead of copy change manually

for more info https://dart.dev/null-safety

for null safety use ? after variable like var a? and while using the variable use ! after variable , like : if(a!){}