Flutter Error : The left operand can't be null, so the right operand is never executed. Try removing the operator and the right operand

Question

I am trying to build my flutter project but I get an error while using the ternary operator. I am using the ternary operator because it would check if the first statement is null, if it is null then it's going to use the second statement. This helps me with my code but I am getting an error, it states :

The left operand can't be null, so the right operand is never executed. Try removing the operator and the right operand.

Example:

value: initial_value ?? current_value

Code :

class SettingsForm extends StatefulWidget {
  @override
  _SettingsFormState createState() => _SettingsFormState();
}

class _SettingsFormState extends State<SettingsForm> {
  final _formKey = GlobalKey<FormState>();
  final List<String> sugars = ['0', '1', '2', '3', '4'];
  final List<int> strengths = [100, 200, 300, 400, 500, 600, 700, 800, 900];

  // form values
  late String _currentName;
  late String _currentSugars;
  late int _currentStrength;

  @override
  Widget build(BuildContext context) {
    MyUser user = Provider.of<MyUser>(context);

    return StreamBuilder<UserData>(
        stream: DatabaseService(uid: user.uid).userData,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            UserData? userData = snapshot.data;
            return Form(
              key: _formKey,
              child: Column(
                children: <Widget>[
                  Text(
                    'Update your brew settings.',
                    style: TextStyle(fontSize: 18.0),
                  ),
                  SizedBox(height: 20.0),
                  TextFormField(
                    initialValue: userData!.name,
                    decoration: textInputDecoration,
                    validator: (val) =>
                        val!.isEmpty ? 'Please enter a name' : null,
                    onChanged: (val) => setState(() => _currentName = val),
                  ),
                  SizedBox(height: 10.0),
                  DropdownButtonFormField(
                    value: _currentSugars ?? userData.sugars, <--Error : **userData.sugars**
                    decoration: textInputDecoration,
                    items: sugars.map((sugar) {
                      return DropdownMenuItem(
                        value: sugar,
                        child: Text('$sugar sugars'),
                      );
                    }).toList(),
                    onChanged: (val) => setState(() => _currentSugars = val),
                  ),
                  SizedBox(height: 10.0),
                  Slider(
                    value: (_currentStrength ?? userData.strength).toDouble(), <--Error : **userData.strength**
                    activeColor:
                        Colors.brown[_currentStrength ?? userData.strength], <--Error : **userData.strength**
                    inactiveColor:
                        Colors.brown[_currentStrength ?? userData.strength], <--Error : **userData.strength**
                    min: 100.0,
                    max: 900.0,
                    divisions: 8,
                    onChanged: (val) =>
                        setState(() => _currentStrength = val.round()),
                  ),
                  ElevatedButton(
                      style:
                          ElevatedButton.styleFrom(primary: Colors.pink[400]),
                      child: Text(
                        'Update',
                        style: TextStyle(color: Colors.white),
                      ),
                      onPressed: () async {
                        if (_formKey.currentState!.validate()) {
                          await DatabaseService(uid: user.uid).updateUserData(
                              _currentSugars ?? snapshot.data!.sugars, <--Error : **snapshot.data!.sugars**
                              _currentName ?? snapshot.data!.name, <--Error : **snapshot.data!.name**
                              _currentStrength ?? snapshot.data!.strength); <--Error : **snapshot.data!.strength**
                          Navigator.pop(context);
                        }
                      }),
                ],
              ),
            );
          } else {
            return Loading();
          }
        });
  }
}

'MyUser' data class :

class UserData {

  final String uid;
  final String name;
  final String sugars;
  final int strength;

  UserData({ required this.uid, required this.sugars, required this.strength, required this.name });

Screenshot Update

Solution

You've declared the _currentSugars variable as a non-nullable string. So you are explicitly stating that it cannot contain a null value. By adding late keyword to that, compiler can't ensure the null-safety constraint at compile time.

As per late-variables

The late modifier means “enforce this variable’s constraints at runtime instead of at compile time”

So instead of declaring it as:

late String _currentSugars;

Declare it as:

String? _currentSugars;

Comments

[Yeasin Sheikh]

how about using _currentSugars!=null? _currentSugars: userData.sugars

[Midhun MP]

You declared _currentSugars as non nullable. Declare it as String? _currentSugars;

[Mike Osborn]

@YeasinSheikh Yeah but it show blue line relating unnecessary null comparison : "The operand can't be null, so the condition is always true. Remove the condition."

[Yeasin Sheikh]

its; ok to use like this, but message is showing because your currentSugars will never receive null value, also you can remove conditional statement.

[Mike Osborn]

@MidhunMP it says: "Conditions must have a static type of 'bool'. Try changing the condition." and the same error: "The left operand can't be null, so the right operand is never executed. Try removing the operator and the right operand."

[Midhun MP]

@MikeOsborn Could you please show how you changed the condition and how you declared that variable now ?

[Mike Osborn]

Yea see my update screenshot

[Midhun MP]

@MikeOsborn I mean variable declaration. Instead of late String _currentSugars; declare it as String? _currentSugars;

[Mike Osborn]

Bruh it also works

[Mike Osborn]

Anyway thanks for the help

[Mike Osborn]

@MidhunMP can you also answer this question if you want to.stackoverflow.com/questions/68449448/…

[Mike Osborn]

Can u help me with this stackoverflow.com/questions/68493717/…