UsageLimitWidget should not load usage for unsaved entities

When creating a new promotion, the Add new Coupon button fails.

The log gives:
TypeError: Argument 1 passed to Drupal\commerce_promotion\PromotionUsage::getUsage() must implement interface Drupal\commerce_promotion\Entity\PromotionInterface, null given, called in /Applications/MAMP/htdocs/uctd8/web/modules/contrib/commerce/modules/promotion/src/Plugin/Field/FieldWidget/UsageLimitWidget.php on line 82

It looks like the newly created Coupon entity has no Promotion ID set yet, when getUsage is called.

A solution would be to test for this before calling getUsage in UsageLimitWidget.php:

if ($promotion = $entity->getPromotion()) {
    $usage = $this->usage->getUsage($promotion, $entity);
}
else {
    $usage = 0;
}