show method
Implementation
Future<T?> show() {
assert(!_transitionCompleter.isCompleted,
'Cannot show a $runtimeType after disposing it.');
if (onWillPop != null) route?.addScopedWillPopCallback(onWillPop!);
overlay!.insertAll(_overlayEntries = _createOverlayEntries());
_controller.forward();
_configureTimer();
_configurePersistent();
return popped;
}