Form submission handler for the Preview button.
public function submitPreview($form, &$form_state) {
// Rebuild the form with a pristine $view object.
$view = $this->entity;
// Attempt to load the view from temp store, otherwise create a new one.
if (!($new_view = $this->tempStore
->get($view
->id()))) {
$new_view = new ViewUI($view);
}
$form_state['build_info']['args'][0] = $new_view;
$form_state['show_preview'] = TRUE;
$form_state['rebuild'] = TRUE;
}