How do I solve the 'Failed assertion: boolean expression must not be null' exception in Flutter

Question

I was working on a Flutter application and kept receiving this error String in the logcat.

Failed assertion: boolean expression must not be null

Here is the code in question:

@override
Widget build(BuildContext context) {
    return Center(
        child: ListView(
            children: <Widget>[
                TextField(
                    controller: controller,
                    decoration: InputDecoration(
                        hintText: "Type in something..."
                    ),
                ),
                RaisedButton(
                    child: Text("Submit"),
                    onPressed: () => addString(),
                ),
                Flex(
                    direction: Axis.vertical,
                    children: (list_two = null) ? [] :
                    list_two.map((String s) => Text(s)).toList() 
                )
            ],
        ),
    );
}

What is causing the problem?

PGMacDesign · Accepted Answer · 2018-06-26 22:57:24Z

17

The solution was a simple one, this line here:

            Flex(
                ...
                children: (list_two = null) ? [] :
                ...
            )

Needed to have the children comparison be a boolean, which requires 2 equals signs.

            Flex(
                ...
                children: (list_two == null) ? [] :
                ...
            )

While using Android studio and writing in Java, this would normally throw a compiler error and not run, but while writing in dart with the Flutter plugin (1.0 as of today, 2018-06-26) no compiler error is shown and we instead see a runtime error.

answered Jun 26, 2018 at 22:57

PGMacDesign

6,3248 gold badges44 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

lrn Over a year ago

The issue here is that the type of the expression is assingable to bool because in Dart, even booleans are objects and therefore nullable.

PGMacDesign Over a year ago

Yup, though I expect quite a few people coming over from Android development will have the same issue as looking for compiler errors is second nature while using intellij and not seeing them may cause a dev to ignore it as a possible problem.

Akram Chauhan Over a year ago

However this solution didn't worked for me. but make sure you initialize whatever you are comparing. for example if( a == true ) in that case always initialize a with true or false ... else may be possible that a never get assign and app will break.

Seriously Over a year ago

To avoid errors like these use Yoda Conditions.

Farman Ullah Khan · Accepted Answer · 2021-05-29 11:29:07Z

1

this error often caused by when a bool variable is checked using = operator

answered May 29, 2021 at 11:29

Farman Ullah Khan

811 silver badge1 bronze badge

Collectives™ on Stack Overflow

How do I solve the 'Failed assertion: boolean expression must not be null' exception in Flutter

2 Answers 2

4 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related