15

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
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3 Answers3

15

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.

PGMacDesign
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    The issue here is that the type of the expression is assingable to `bool` because in Dart, even booleans are objects and therefore nullable. – lrn Jun 27 '18 at 06:03
  • 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. – PGMacDesign Jun 27 '18 at 13:41
  • 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. – Akram Chauhan Jun 11 '19 at 07:11
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    To avoid errors like these use [Yoda Conditions](https://en.wikipedia.org/wiki/Yoda_conditions). – Seriously Mar 29 '20 at 12:53
2

This problem occurs when one of your defined boolean type variable is not initialized with default value, and you try to use and assign it somewhere as value. example maybe you have you bool isEnabled ; is not defined as should bool isEnabled = false; or bool isEnabled = true; and you try to use it like readOnly: isEnabled,

To avoid these to ensure isEnabled won't be null. Here is a kind of example of how we can avoid these

bool isChecked = false;

Checkbox(
  value: isChecked,  // initialized with `true/false` also avoids the error,
  onChanged ...
)
Paresh Mangukiya
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0

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

Farman Ullah Khan
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