AH
$pageWidget = PageWidget::where(['page_id' => $savedPage->id, 'widget_id' => $widget->id])->first();
если $widget->id может быть NULL ?
use Illuminate\Database\Eloquent\Builder;
$pageWidget = PageWidget::query()
->where('page_id', $savedPage->id)
->where(function (Builder $builder) use ($widget) {
$builder
->where('widget_id', $widget->id)
->orWhereNull('widget_id');
})->first();