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;
});
}