void debugWidgetBuilderValue(Widget widget, Widget built)

Asserts that the built widget is not null.

Used when the given widget calls a builder function to check that the function returned a non-null value, as typically required.

Does nothing when asserts are disabled.

Source

void debugWidgetBuilderValue(Widget widget, Widget built) {
  assert(() {
    if (built == null) {
      throw new FlutterError(
        'A build function returned null.\n'
        'The offending widget is: $widget\n'
        'Build functions must never return null. '
        'To return an empty space that causes the building widget to fill available room, return "new Container()". '
        'To return an empty space that takes as little room as possible, return "new Container(width: 0.0, height: 0.0)".'
      );
    }
    return true;
  }());
}