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

Issue

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?

Solution

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 By - PGMacDesign